I have purchased a copper head gasket from Speedy Spares for a modest sum for my 1725 SV but have since read that a Payen gasket is much better. Could someone who is in the know advise wether my cheap one will perform adequately or should I push the boat out and get a top quality one

I have heard the same view as you, but I really have no evidence.
But are you prepared to bet a day's work that it isn't true to save £20?

The gasket you find in the original "Rootes" package, who has made them?
All the gaskets I have for the alloy head are the copper/tin type. For the iron head, "tin" type.

The gasket sometimes wants to "creep" away. I suppose it's because they are living between iron and alloy.