This lawyer was in Berlin.
ChrisN has got it pretty much right. Glider pilots aren't normally trespassing because necessity (i.e. no choice) is a defence to trespass. For field landings we only have a choice of possible fields (if we've done it right), but no choice but to land in one of them.
If you could elect to stay flying, you're a trespasser. Assuming you don't cause damage to land or livestock, there's nothing to sue you for. However, the
PR damage to GA is the real worry.
I'd suggest contacting the landowner and telling the romantic story - all the farmers I know are softies deep down, so I think you'd have a good chance. As you'll want to view the site on foot (gliders are designed to cope with rather more roughness in the terrain than powered GA, if what I read about the consequences of forced landings is correct) you can find out who that is. If you add that you're fully insured - and you have checked that out, havent you - that should cover all the farmer's concerns.