OK, so thanks to Incey 1966 I managed to get the the parts from David Manners. However there are two parts to each rubber isolator, one of them being the rubber bush and the other being a plain rubber washer that goes on the other side. The part number for the rubber washer is UKC1090J, which are no longer available!
David Manners had 10 of the FRC7418J which are the same thickness as the UKC1090J so I have bought 8 of these from him with the intent of cutting off the shouldered 'bush' part to make them into the plane washers.
Thanks for all your inputs, I hope these part numbers are of some use to others who I hope will have more success than me when trying to locate them.