Right, so the mystery has been solved. Bear with me on this one...
Firstly I contacted a guy who was rebuilding a B25 in Michigan, this was before I posted the parts on here. He thought it was a vacuum pump that run the avionic but couldn't be sure as there were no part numbers. It's always good to get a second opinion, so I posted on here where YGBSM suggested I contacted the B25 world experts.
A Skype phone call from the UK to the US and contact had been made. An email later and the mystery grew, it appeared as though the pump was a hydraulic one but of the type that belonged to a Merlin powered plane, not a B25. That made no sense as the area the parts were found in is far too small and remote for any other air crash to have happened there. Also, local records back up that there was only one air crash in that area. We then thought that maybe the RAF had swapped the pump for some reason.
Another couple of emails back and fourth and I asked if they had an image or drawing of the pump it should have been. Trawling through the parts manual the pump shown above was miraculously found, it's labelled as an 'alternative' part. One that can be fitted optionally. In the 40 odd years the guys have been rebuilding B25's they've never seen that pump fitted, hence why it threw them totally.
The hydraulic pump sits directly on the back of the engine, so we're in the right area for the crash site. We still don't know what plane this was from, as two collided in mid-air. Hopefully with further investigation around those known locations we'll be able to determine if it was from my great uncles plane, or the one that they collided with.
The second, more corroded pump with just the single large connector is still a bit of a mystery. Even though the outline of it is identical to that of the hydraulic one, it's too badly corroded to make out what it could be.