Finally got all the parts to build my own 'grease gun hydro pump' and it worked a treat.
Thanks a million to the people who posted those 'how to' instructions. There doesn't seem to be any leaking so fingers crossed. I was surprised how little fluid it took to pump it up but this means my 4litre tin will be on hand for future use.
Unfortunately this hasn't stopped the knocking noise which I thought was due to the car riding on the bump stops. But I think it's coming from the steering, I will post a new thread and ask the question.