Question Author
Thought of an elastic band stanleyman but the fan is in a kitchen and I didn't think it would last long with heat and steam.
Also thanks to Clanad. Unfortunatly our DIY in north London only has blister packs of best selliing items. Anything out of the ordinary, forget it !
Solved the problem late last night. Had an old VCR in the shed waiting for disposal. Dismantled it , found a spring and with the help of a couple of pairs of pliers and a bit of bad language managed to get almost a perfect fit. Works better than the old one.
Again thank you both for your suggestions, they were very much appreciated.