Hi Haz,

3 very good points

Definitely a genuine Bosch pump, but no non return valve supplied with it, and so not replaced. I think this is a very good point and is something I'll look at straight away

Yes, accumulator is something I've considered but I would think problems here would manifest themselves further upstream, metering head' for example with a distinct effect on running. As I say, it runs perfectly

Will test voltage at pump with problem evident, but you don't here it slow down at all so don't think there is a problem. Any voltage drop can be heard, for example a small effect on the pump when the fans cut in

Thanks