Yoggy,
Pumpkinseed are as Mark rightly says from North America.
Scientific Name: Lepomis gibbosus
Lepomis, from the Greek, "scaled gill cover"
gibbosus, from the Greek, "wide margin"
Common Name(s): Pumpkinseed, Common sunfish, yellow sunfish, punky.
To import these days you need a DEFRA licence (Section 30) however, there are quite a few stillwaters where these were stocked in the hope that they would thrive in some Carp lakes. Mostly the ones you catch in the South are very small and a real nuisance.
I know of one small lake in Hampshire where they are very prevalent, in fact to the point where 'natural' baits are now out of the question to use.
The USA record is: 1lb 6 ounces.
They do have very sharp spines so handle with care