Thanx guys, found the problem. The reservoir hose flange bolts weren't fully tight onto the pump housing, must have vibrated loose like Lee said. Tightened them up, filled up the fluid, bled the system and all is well with the world again.
When the weather is warmer i will remove the flange and fit a new seal, and fit shakeproof washers and threadlock to stop it happening again!
Thanx again :D :D :D :D