Huw, you will most likely need to clean up the damper paintwork and see if you can find the make, reason being the inner diam that fits over the pin will be standard but the outer diam of the rubber will most likely be specific to that top ring on the damper
When you have the size you could try an online search or ring SSL (was Suplex) as Dan is very helpful
https://www.suspensionsupplies.co.uk/morgan
