I have a working heater now. After my experiments with replacing the component itself - tried a n-channel MOSFET- and it too frying - I gave up and put a new board in and once more that too fried straight away. I thought my solenoid valve set was OK as it was brand new earlier in the year, but I took it out anyway and found it to be soggy and shorted - hence probably causing the excessive current and thus the blown component. In the end I refurbished a previous solenoid set, put that in and then got another replacement circuit board and I now have a sensible heater. I intend to monitor how much current the solenoid actually takes and fuse that wire with a sensible fuse to prevent further blowing of the expensive circuit board! (Job for the summer!)