A common problem with spitfires is a little slop in the u-joints allowing them to move back and forth as the axle turns. It's more of a clicking noise than a knocking but otherwise matches your symptoms. I know that you've just replaced the u-joints but if it wasn't really difficult to get the circlips in, you probably have play. You can eliminate the play by making a shim out of some thin scrap metal (a soda can works just fine) and putting it under the circlip. You may also be able to get some thicker circlips, some j-joints come with the thin ones and some with the thicker ones.
Good luck!