GTD 10W/40 would be ideal mate, 15/40 would be a touch thick when the engines cold and may cause start up wear. 15/40 might be ideal if your in a hotter place but not in this country mate.
Millet Castrol GTX 10/40 is petrol engine oil mate. some diesels do have petrol engine oil in them but to put a diesel engine oil in, will not do any harm.
Multifleet is a monograde engine oil which makes it suitable for both petrol and diesels. It would be o.k but the Castrol stuff is better!! not that I am biassed in any way