I've just installed a Stuart Turner Monsoon Twin pump (3-bar, positive head) and it's misbehaving: starts fine, but the hot side will not switch off in normal use. Instead, it continues to run unless either I cut the power, or turn off the isolating valve.
Looking at the ST website suggests this may be either a dodgy reed switch or PCB (switch is in the right place and seems to test OK with a fridge magnet), or a leak in the pipework. None of the outlets are letting by, and I don't think I've got leaks elsewhere: nor does the pump restart of its own accord.
Being, um, parsimonious, this is a second-hand unit but at least cosmetically is in perfect condition. I've got no reason to believe the pump itself is faulty but suspect it's something about the interaction with my pipework: I'm pumping the whole house (well, bathroom and kitchen) not just a shower.
I'll call ST after Easter, but meantime any advice about what to try, test or change would be appreciated.