Hi Nigel
This is interesting as my Sierra Brake Master Cylinder bought at the same time as yours has also failed.
And like you I searched far and wide to find a direct replacement without success.
I have bought a repair kit containing several seals and hope this solves the problem.
Interestingly Peter Edwards managed to find a replacement being offered on eBay -
https://www.ebay.co.uk/itm/334299608631