It's is possible, indeed probable, that the original Pimm's Cup served in the eponymous London bar was in fact based on old tom gin rather than London...
Hmmpf... Which Old Tom are you all using? I went into my attempt with this armed with Ransom Old Tom which, of course, dominated to my delight. Barr Hill (Vermont) Tom Cat (delicious) would probably be comparable to Ransom, just plus honey drippings. Is Hayman's better? Turn me on to a new UK brand please.