Not a one hit wonder, but just an honerable mention(mostly because I can't think of any other one hit wonders that are also about masturbation). http://www.youtube.com/watch?v=yd4cW4hZqVo
OK, as we've stooped this low, this one reached 20ish in the UK charts (and was inspired by George Formby!)
It was recorded in a BBC studio ... and promptly banned by the BBC. YouTube - Ivor Biggun' - Winkers Song
Whoa Ray it appears you have an unfortunate gastro-inestinal malady which is forcing the regurgitation of old songs. Outstanding!
I don't know about the Grandma song but it actually hit numero uno in the UK! Shaddup You Face hit the top in UK as well but in the USA it only made it to 56.
I will bust myself here so someone else doesn't get all happy. Yvonne Elliman was actually a two hit wonder. She hit in 77 with the BeeGees on the Saturday Night Fever soundtrack with If I Can't Have you. She also did the background vocals for Eric Clapton on I Shot the Sherriff.