IME it doesn't really work. I used two jars of Damson jam and it gave no noticable damson taste, whereas Poacher's just fills the room up with damsons.
I'd say you'd have to use somethign silly like 8-10 jars to get anything, and then of course we're talking serious amounts of sugar.
I personally think that Badger use Damson extract to flavour Poacher's, but failing that next year i'd say get perhaps 2 kilos of fresh damsons, boil them to mush, strain the juice out through muslin and use pectic enzyme.
Using Damson Jam???
Re: Using Damson Jam???
When I made rhubarb wine, I found that freezing the rhubarb and then letting it thaw really brought out a rhubarb taste. Some recipes recommended doing this step twice. Dont ask me why but it really worked. It might work for you Damson extract....