As the current wing appears very similar to the venerable Pegasus "XL", and is presumably a pretty docile, slow flying example, you are going to have problems finding a Section "S" approved equivalent. No current UK built flexwing would be comparable (speed being the thing these days), and all the existing XL's are getting very long in the tooth. One of the French UK legal trikes has a similar wing, but I don't think there are many of them about.
I don't think that "docile" is the issue, I think that stall speed is probably the issue.
I imagine that if you asked P&M nicely they'd still build an XL for you - they're certainly still supplying spares, and it's easy and cheap to buy a second hand one. But, it hasn't got the MTOW you'd need for this.
I'd go for a Quantum - it's got the structural lifting capacity, good handling, stalling speed only a few mph above the XL. A quick look on AFORS shows a Quantum 912 for £11k and a 503 for £5k.
Offhand if you had this, the £5k Quantum 503, so long as it's got the 409kg structural mods would maybe need a few hundred worth of parts to marry it all up, after which it's just skilled work - and you'll get a few thousand back selling the unwanted 503 engine and trike parts.
G