Thanks for the helpful responses. Yes, a 2.5 diesel and I had been bleeding the system (what a pain in the ass!) pretty much every time I topped up. She was only losing a very small amount of water, but I was really trying not to put her under too much stress. I will contact the other forum user ...

Finally got my Bongo back on the road five months ago after a long time laid up. Had persistent problems with losing water from a hard to find leak, so was topping up regularly, then disaster - a roadside meltdown and poor old Freda Bongo is off the road again :(

I was hopeful it might be the pump ...