It looks like a Slingsby with a VW air cooled engine shoved in the front....
Correct. It's a modified Slingsby T. 31B Tandem Tutor (RAF Kirby Cadet Mk. III) glider. It has a simple undercarriage instead of the original skid & the fuel tank fits nicely in the covered over front cockpit.
This is what it originally looked like. Not much difference except for the engine.

It's only just been completed & this was its first outing.