I have now got a new O ring for the ram which is 5.3mm instead of 5mm diameter. The O ring fits the piston beautifully and the ram fits into the cylinder OK. It is of course much more rubbery than the old O ring. We will see how it seals. The advice the I received from the hydraulics supplier was that an O ring will probably seal better that a U ring as the ring slot in the piston was not designed for that type of seal. I will take their word for it.
I have also sanded enough from the nylon bushing to allow the ram to travel freely. I am making progress.