UPDATE
Got second hand shock from ebay. Put it on and had a little ride ride. Noticed tyre wear on rear was different after only a couple of miles. Took the missus out last night (totally dreading it in case it didn't work). She said it was a 100% improvement. Yippeeeee. Got back and looked at rear shock and it looks like it is nearly bottoming out. Will adjust the preload a bit more which should sort it out. So after much research and going by tyre wear it looks like the shock was knackered. happy bunny now. Might invest in better shock or stronger spring in the near future now i know its not the bike or the missus. Also looks like i need the fork oil changing as it still has a second bounce and it is only 1/4 turn out.
