Spring Harvest used to be huge. It was the go-to, agenda-setting, worship-leading event of the evangelical Christian year. Held over three sites, in the mid-90s it was attracting more than 70,000 people. Times changed, though. New kids arrived on the block: New Wine in 1989, Soul Survivor in 1993, Momentum in 2004. There was an uncomfortable split on theological grounds with Spring Harvest's highly conservative Word Alive spin-off in 2007; New Word Alive now attracts around 4,000 guests.

Spring Harvest went through years of declining attendance, reducing the venues to two – Minehead and Skegness – and cutting the number of weeks it offered. Last year it did something unprecedented: while it's raised more than £10 million for charities through its offerings, 90 per cent of the 2014 offering – £273,000 – went to support the work of Spring Harvest itself.

Looking purely at the numbers, it's a story of decline – though that doesn't take account of its continuing ministry and the experiences of those who've attended year by year. But what does the future hold for the conference that created the mould for its later imitators and offshoots, and what does it feel like today?

Spring Harvest is an expensive thing to run, with three weeks at Butlins in Minehead and one in Skegness. Financially, though, it's in a better place. Organisers hope that diverting last year's offering was a one-off: it's developing a "partner base" of regular givers with the aim of raising £300,000 a year by 2017. In terms of turning attendance round, early signs are good, with claims that the decline has been stemmed; last year it was about 17,000 at the two venues, while this year it's closer to 20,000. Numbers, though, only tell part of the story. I asked Malcolm Duncan, minister of Gold Hill Baptist Church and Chair of Spring Harvest's planning group, about where the conference was going and how it was going to get there.

He places it firmly in the centre ground of evangelicalism. It's charismatic evangelical, and one of its strengths is that it's pan-denominational, not biased toward any particular Church group – unlike New Wine, for instance, which is Anglican-dominated. It's also egalitarian in its approach to gender rather than complementarian. Spring Harvest also deliberately chooses a balance of Caucasian and non-Caucasian leaders, and is committed to including disabled people alongside non-disabled. (Though Duncan adds that the choice of speakers is primarily determined by what they have to bring: "We don't say, 'We have to find a woman for this.' We want to find the person who will speak into the theme most effectively.")

Neither is he interested in competition with other conferences, he says, theologically or otherwise. "There is far more that unites us than divides us. We need to celebrate the cross at the centre of evangelicalism."

This commitment to unity, he believes, is one of Spring Harvest's strengths. "I think many evangelicals are searching for a place of unity," he says. "That's doesn't mean we're all the same, but we stand together for the sake of unity and we find ways of having difficult conversations about various issues of theology without those conversations becoming offensive or accusatory or undermining of one another."

In terms of the experience of guests, he says: "The things that birthed Spring Harvest still drive us. The evening progamme, the way we deliver things – we've enabled more space to hear God speak, new speakers, new approaches. There's a sense of excitement. We want it to grow and we believe it will.

"We've sought to build on the good stuff that's happened previously. But we have introduced changes and we have asked what Spring Harvest needs to look like in its next season."

For visitors I spoke to, the formula seems to work. Louise Seeley from Cheltenham came with a group of 50 from her church for her first time since she was a child. She says her children have really enjoyed themselves, and she's attended the parenting course run by Rob Parsons and the Christians in Politics debate about whether voting is worthwhile. Angela Constantine from Sutton Coldfield has come to Spring Harvest many times during the last 20 years. She's a Methodist local preacher and enjoys the worship and the Bible teaching. "It's a taste of heaven for us," she says. Andrew Jervis from Sussex is the head teacher of a primary school who with his wife Christine runs Spring Harvest's under-5s ministry. He's here with his family. "People think we're mad," he says. "We finish school on Friday and come rushing down here – but I wouldn't want to be anywhere else."

Three people are hardly a scientific sample, but what they said seemed to reflect the general atmosphere on the site: it was happy, relaxed and peaceful. Reflecting on my experiences of being here 20 years ago, I'd say it's a lot less intense and frenetic: the sense of being overwhelmed by the expectation that something spiritual had to happen was missing. I think that's healthy, and reflects a maturity in the charismatic movement as well as perhaps in Spring Harvest itself.

Like all such conferences, Spring Harvest offers a range of seminars and meetings aimed at slightly different audiences. This time, with the overall theme Immeasurably More, morning seminars grouped under the titles '--- More' looked at the work of the Holy Spirit. Engage More was about what an empowered life looks like at home, work, church, in society and in the world; Chew More about the theology of the Spirit; Drink More was about how to "wait on God, rest, soak and receive more from the Spirit" and Lead More was about exploring the impact of Holy Spirit on church leaders. Was I blessed? Well: not invariably. I don't identify as a charismatic evangelical, and found myself wondering, heretically perhaps, what there was of real theological substance and practical value in a discussion of the different prepositions used to describe the way the Spirit relates to us. And I wondered too whether speakers really had a clear idea of what they meant when they used language about "going deeper with God", for example. Perhaps this all made more sense within the charismatic evangelical culture – but I would have pressed them to explain it better to those who are slightly off to one side of it.

Other sessions I enjoyed more. The Christians in Politics debate, for instance, which brought representatives of diffferent political parties together to discuss whether 'This house believes that the 2015 general election will not make a significant difference to the crucial issues affecting our nation' was very revealing. OCCA lectures addressing various apologetic questions were satisfyingly stimulating. Sung worship, both mainstage and in the smaller Hub venue, was expertly musical and the congregation was mainly engaged with it: it's a high point for many who come from smaller churches which can't muster such resources. 

So, marks out of 10, if that's appropriate? An easy eight. Spring Harvest has a lot going for it – and if its organisers continue to think hard about what they're offering and how they can broaden its appeal, there's no reason at all why it shouldn't continue to grow and prosper.