replaced shower, now pump doesn't kick in

[Monnie]

I have replaced a 17yr old Hansgrohe BIV shower (min 0.5bar) with a Mira Atom bar valve(0.1bar) However when I try to use it there is a whirring sound in the valve, a trickle of water from the showerhead and the pump (Stuart Turner Showermate( also 17yrs old) doesn't kick in unless I take the shower head to floor level, (even then it doesn't always work) or turn the temperature control to cold. With the bar disconnected, the water flow through the pipes looks fine, in fact slightly stronger on the hot side.
I have a gravity fed system - cold tank in loft and hot tank next to shower in airing cupboard.
I could call out a Mira engineer but if the problem isn't caused by a defect with the Mira valve, it will cost me £70+ for the callout.
I would be very grateful for any suggestions as to the cause or solution to the problem. Thanks

 

[kbdiy]

Have you followed the commissioning instructions that come with the shower? The incoming hot temperature needs to be set by moving the position of the temp control until you achieve a good balance of temperature. Just wondering if the hot is set too low and not generating sufficient flow to trigger the pump.

 

[Monnie]

Sorry I must have sent it twice.

Yes we did adjust the temperature because when we first tested it the temperature was set so low that it needed to be on the maximum setting to achieve a warm, not hot shower.