So i did the job too (that time of year i guess!)
i tried shortening a socket but i just still couldn't get it to work out for me, the bolt was at the wrong angle maybe or the ratchet was slightly too bulky maybe? either way....
i went to halfords and brought the adaptor set that fits ratchet spanners, you ideally really need the flexi head ones.... and the kit costs £10 odd if i recall (forget if that was before or after trade card) and the ratchet spanners aren't that cheap so unless you really want them, maybe borrow off a friend ect or the job will cost you £60 in tools plus parts!
i got a genuine VW thermostat off eBay with an O ring for £20 delivered, stamped VW and Audi ect. which was cheaper then my local dealer with discount.
also notice i have cut this 10mm socket down which was my first attempt using a socket and wobble bit that failed
that lets you get down there (note the bend on the ratchet head is needed i think)
another view
so got that out and lost a little coolant from the plastic pipe, at first it wouldn't only come out 5mm, you need to fish out the O ring which is normally stuck in there holding the thermostat in place and the stat is sort of held in the black elbow.... i just pulled the elbow in the end and it came away, then removed the stat from the block and about 3 litres of coolant came out.
on refinement, ideally new O ring, and carefully note how the stat is located in the engine orientation wise.
it sits in the elbow and is locked in by turning 90 degrees, then the two get put back on the engine as one, and two bolts snugged up.
i then refilled with water and checked for leaks and a bleed drive ect, and it gave my 90 degrees temps finally!
i then drained the coolant and filled with G13 from VW and all has been well since.