Presumably SEP hours were flown on the integrated course leading to MEP flying, an MEP CPL skill test and MEP/IR skill test, but no SEP skill test?
Anyhow, it's straightforward enough - training as required with an FI or CRI, then an SEP class rating test. Pretty much any school can do that, probably in something rather cheaper and simpler than you've formerly flown. Hopefully an opportunity to discover the fun in flying that you may not have observed much of to date. Just remember that recreational GA flying is a very different place to the airline training you've done to date, so be prepared to do some completely new learning - including learning how to be truly independent in an aeroplane, and learning how to enjoy yourself in the air.
Where in the UK are you? The odds are that somebody on Pprune can sort you out.
I'd be surprised given your training and qualifications if the whole thing costs you more than £500. Quick SEP refresher, type & local brief with an instructor, then a class rating skill test and whatever CAA charge to amend your licence. 2 hours flying and a test fee probably, maybe a little more if you've not flown for a year or so or have never done anything resembling private flying.
G