Thanks all for your advice. It's the starting problem I'm still stuck with. I will follow Jules advice from older thread here:
https://www.the75andztclub.co.uk/for...d.php?t=143059
... and start again from beginning. The tanks is showing a quarter full and I suppose I can't assume that in tank pump is actually pumping just because I can hear it working.
On this point, can anyone explain why the UBP can cope without help from the the in-tank pump when the tank is full, but not when the level in the tank falls?
will post again when fixed.