griffin are fab. Personally prefer them to Wintec as they're more comfy and the wintec doesnt seem to come up as wide.
I have a griffin pony club and it seems to fit anything round or table top backed.
My daughter has a 13.1hh highland/connie type. No withers, huge shoulders. Her old saddle would go forwards and sideways even though it was extra wide . I bought her a saddle company saddle and had it fitted and it doesn't move an millimetre. I've been really pleased with it plus its very comfy to ride in too.
Have a look at the Ideal VSD on the Europa tree. I have one for my roly poly NF who's old saddle used to slip side-ways something rotten. I've found a non-elastic girth helps alot too.
I found synthethic saddles are too light even if they fit perfectly, but if it's a synthetic you are wanting try the Thorogood Jill Thomas Endurance saddle.