Steve
It is hard to day without seeing it but sounds like a plum leaf curl
aphid. It is a common pest that affects all plum - gages & damsons.
Picture on the RHS site at
http://www.rhs.org.uk/advice/profile...plum_aphid.asp
Does it look anything like this?

Have recently seen sloes that were hard and shriveled looking like
they were dried last year - but inspection with a hand lens show the
presence of a small white / translucent spider - like a red spider
mite without the colour. Might be worth a look at the fruit

Regards

Clifford
Bawtry, Doncaster, UK