I decided to tackle taking the control block off of the pump and seeing if there is any dirt stuck in it. It's not a job I've been looking forwards to, but actually its fairly simple to get the block off.
Unfortunately it was not simple getting the spool caps off from either end of the control block, as it seems that they have been fixed in place with some kind of thread lock!! So that is the end of that, I blew out the orifice with an air line and put it back on the pump. Wast of time.