Plumbing Questions? Ask a Plumber for Answers ASAP
I gave this some thought, after bringing in a heating and plumbing company for another view (who recommended I spend over £800 reverting the system to what it was before installation of the monsoon pump and new shower mixer), and decided to go with my own 'fix'. I will have a local plumber install a concealed cold shut off valve (chrome lever to match the other fittings) just under the shower mixer.
That means we simply turn this on and off when we use the shower. No more overflow.