1856 Florin Fake? Grade?

Simple test you can do at home:
Comparative ping tone test against another known to be genuine Gothic Florin.
Need to go to a coin dealer if you haven't got another Gothic Florin, to do the test.

Accurate weight needs to be found, to nearest 1/100th of a gram. A coin dealer or a pharmacist may be able to help you.
XRF test. Need to go to a bullion dealer or scrap gold jewelry buyer for that. They may charge a fee.

From the pictures the coin seems to have a 'pasty' appearance suggesting a pressure cast fake. Fancy rim border above Queen's head is suspiciously weak. The cross on top of the orb on top of the crown is very badly out of shape.
These coins tend to have some value in this grade, so the temptation for the dishonest is there.

"I think Peter Davies' book on British Silver Coins is the most up to date and comprehensive reference but I'd be interested in anything more recent."

I think the 2015 edition of "English Silver Coinage" (ESC for short) is more up to date. This is the sixth edition updated by Maurice Bull and published by Spink. Readily available through Amazon.

Oh thanks - I do have the 6th edition I think but didn't look too closely at it as the edition before that at least covered much fewer varieties than Peter Davies' book.