I'm after a master cylinder, and suitable fluid reservoir, to use with my sierra based kit with standard brakes. What have you got?

Something like Brake Master Cylinder Fiat 124, Delphi LM70067 19.05mm Bore is what most people would recommend if your not going the balance bar route. You'll obviously need a remote reservoir (fiat do one - search ebay) with low level switch & IVA acceptable pipe to go with it.

I suggest something more universal/modern:
Fiat Cinquecento/Seicento master cylinder - also 19.05mm bore - but with "modern" M10x1 thread...
DELPHI master cylinder, brakes brake FIAT LM70169