Thankyou for the reply, i would have hoped that when the mechamic was looking for the loss of water (leak as he called it) that he would have checked the water pump as a likely cause? he did do a carbons test and checked the heater matrix so he seemed thourough?

I have been doing alot of googling and one thing came out that seems possible, someone mentioned a faulty expansion cap not holding the pressure so the water simply evapourates over time, i do remember quite some time ago that when i undid my filler cap that some thiN circular thing fell out and i couldnt find it, i didnt think anything of it at the time...

Where can i buy a replacement filler cap please?

I know its most likely not my problem, but at the moment no-one seems to know whats happening to my water/coolant so its worth a shot?