Replacing broken fitting

Maintenance

One of our tenants has broken the shower mixer/control which has resulted in it needing to be replaced.
The bathroom was only refurbished in December 2018 and on speaking with the plumber who installed it, I'm advised that it was a Bristan mixer/control that was fitted at that time.

I have asked our plumber for a price to supply and fit and new one and have communicated this to the tenant as well as advising her that on this occasion, we would be willing to pay towards half of the cost.

She has replied back to me by email stating that she's been doing her research and has found a cheaper mixer/control online (which isn't a Bristan and doesn't appear to be any particular make - it's on the Victoria Plumbing website) The mixer/control is %C2£20 cheaper than what our plumber has quoted.

My question is, where do we stand as landlords about insisting that a like for like replacement be installed please ? Surely tenants can't break fittings and then replace them with cheaper alternatives ?

I look forward to receiving your advice.

Best wishes,

Susie