First up, make sure these are really what you want - 1050R20 is a slightly different size to 1050-20. If you hadn't picked up on it, the R means the overall outside diameter is slightly smaller.
Assuming you really want R's I have 5 Continental MPT 80 M+S tyres, with plenty of wear left (one spare looks brand new, the other four are only about 5mm off of new), all in very good nick (no cracking, they look almost new) on some 20" wheels. I'm just up the road from Cambridge.
Drop me a PM and we'll talk pennies.