Yeah, converting the Amber is going to be tough. Muntons has never responded to my questions about what's in the stuff, much less what the proportions are.
You can kind of guess, though. Briess's Amber extract, for example, is mashed with 2-row Base Malt, Caramel Malt 60L, and Munich Malt. How much of each? Guesswork. I'd say 80% base malt, 10% Caramel 60L and 10% Munich. But that's nothing to bank on.