Val's damson pickle

HELP!

Tried making damson pickle today following the recipe. But, having spent a couple of hours peeling and stoning 5lb of damsons we were left with a bowl of what can only be descibed as mush (and probably much less than 5lb of mush).

We used this as per the recipe but only achieved sufficient body to 3/4 fill a 0.5ltr jar (although we have quite a lot of the liquid to top up).

Any idea where we went wrong?