hi-lux surf heater problem
 

the problem being it only gets luke warm at best i've changed the stat checked the level of coolant and bled the system best i can with no bleed screws anywhere, the flap appears to be moving and the valve in the heater pipes is working.
it seems to be a problem they suffer with along with over heating but no one seems to know how to cure it.
anyone got any ideas or even better know how to cure it?