Hi Peter
You are right that 20w/50 is the correct grade. It is not as difficult to get hold of as you may think. Duckhams still do their Q oil and I also found some in my local car accessory store. You may have trouble finding it in places like Halfords but ask in your local store and they should be able to help. If you have an MGB engine, you should be using 20w/50 in there anyway.

Thanks for the replies.
I managed to get some 20W/50 from Halfords.
It is their classic oil and is not recycled as some are.

Thanks again