That explains it, thanks for the tip.
Just to update this thread, I have actually been certified as fit for LAPL as of today and so will get cracking with this process. I have been concurrently liaising with the CAA licensing department regarding LAPL conversion, UK PPLs etc.
For reference (as this process isn't officially published anywhere...), re-grade of PPL to LAPL is £41. Re-grade back to PPL is £73 with no testing necessary (remember this is not initial issue). SRG1104 to be used in both cases, although it's a bit of a fudge how you fill it in as the form is technically for conversion of non-EASA licences...
A UK PPL can be obtained for the cost of £36, using either SRG1102 or SRG1104. Again a fudge, to capture details as jollyrog mentioned. I won't be taking this route as it's simply too risky now with only about 6 weeks left of useful life for such a licence on EASA aircraft.
A lot of hassle for what feels like a really silly omission from the original Aircrew Regulation (PPL not being valid for LAPL privileges with a LAPL medical certificate)!