Thanks GORSQ if that stuff works I will use it on all my rubbers (stop sniggering) interesting product name doesn't exactly roll of the tongue like "Rubber Reviver" would.
Fixed the tensioner thingy, I think there are various types but on mine the loops in the buckle are stitched in I only found this out after trying unsuccessfully to shorten it like a belt buckle. I just ignored the loops shortened it a bit, fed it back thru the buckle and put the pin back, fiddly but doable. Minimax's idea of using velcro to hold the elastic would make it much simpler to adjust if needed.