Pump serves a shower in attic. It is used very rarely about once a month. Normally works fine, so not a installation problem.
When switched on, the shower pipework has a low vibration ( a bit like water hammer) and small amounts of water start flowing, then after a few seconds stops completely. The pump appears to be running and I have bled the air ( there was none) from the pump.
Thermostatic shower valve (On/Off) quite tight, may be significant?