2018 Swift Challenger 560 Shower Mixer Tap

Hi our shower mixer tap has sprung a leak during the extreme cold - even though it was drained.  We have a new unit to replace it but cannot work out how to get the old mixer tap off the grey section in the middle of the shower.  This would seem to be the only access to the tap as the whole shower is a moulded unit.  We can access the bottom of the shower over the wheel arch via a panel in the bathroom and we can see the red and blue pipes we need to attach to but cannot get any higher.

Anybody any ideas how to get the old mixer tap out of the shower to insert a new unit.

Thanks in anticipation - Liz