After a bit of trawling the internet for Damson Crumble recipes, I found it was roughly split down the middle... put them in whole and get the texture of the damsons but have to spit the stones or alternatively stew them first and strain them as you would for jam and lose the texture but don't get the stones.
A typical recipe seems to be a pound of Damsons, about 4 spoons of sugar, a dash of water and a knob of butter under the usual 100g flour, 50g sugar, 50g butter crumble topping.
Alternatives suggest going half and half Damsons and Apples and holding back a little of the flour and using medium rolled oats.
Damsons are in the shops now for the next few weeks and they freeze well.