So I have refitted the head gasket and retorqued the head down.Question is do I have to warm the engine and retorque.I have used a fibre Payen gasket.If this is necessary whats the procedure,Manual doesnt mention it being necessary but just watched a B series engine rebuild and they did do it to that engine.

i don't see any harm in doing that go for it makes sense to me

Not really required but best practice to recheck once it cools down.

1275 engines (particularly non-s) are serial head gasket blowers at the brass plug between cylinders 2 & 3 so any extra precaution you can take is worthwhile.

Rightly or wrongly I always retorque the head nuts on any A series after getting the engine hot, thats the way I was taught 45 years ago

What level of tune? If any.. is it?
IF moderately tweaked AND the head is not warped (too common) then a spray coating of Permatex Copper Coat Gasket Spray on a decent quality headgasket will ensure a permanent seal.
Likely hard to source this inna UK though.
It's $6 a can in Canada.. as reference, if you find some..as some vendors have ludicrous pricings.
IF your engine well tweaked: then Oring the Head.
It's fairly inexpensive and Durable to the point where there is NO conceivable Mod that you can do to that engine that will fail an o rigned head seal.