I do not know what sort of Rover P6 you have, but if it is a 2000 TC, I made a sealing ring for where the O-ring that goes between the hosing and the carbs. I used a mouse pad to make the ring I needed, it works pretty good, but try to make it just a little smaller than you think you need. There is a similar O ring inside my 2000 SC air filter hosing which I need to make in a similar manner one day.
I am not so familiar with the 3500 model, but mouse pad, or perhaps very thick gasket paper might work as well.
I also used mouse pad to make a new rubber base for my reverse light lense.
James.