There are quite a few sources (=some of them written down, some of them by witnesses) who confirm that Stuart's playing wasn't that bad. This story began with Allan William's book in 1975, a source very valuable for one side, but full of myths at the other side.
Xosé
Here's an extract from an interview with Bill Harry which discusses Stu's bass playing and says it wasn't as bad as Allan Williams made out ...
http://www.beatlefolks.com/bharry2.shtml