I think if the 10th letter of your chassis is K onwards then mk3 calipers will fit straight on. What is the letter on yours Dave?
ETKA states late 88 was when the later mk2 calipers were first fitted, so might be able to use mk3 calipers on a late 88 as well.
Check Dave's gallery (if he still has the pic on), if your calipers have the handbrake coming up underneath then mk3 calipers will be a straight swap.