I've been trying out that "Imperial Malt" as a potential substitute for Pale Amber. Not that I know what Pale Amber tasted like or tried to make the stuff to DPBC instructions.
I picked a recipe that I'd used Simpson's Amber Malt in (Fuller's 1845 clone) and replaced the 10% Amber with 75% (!) Imperial Malt. Seems good so far but not ready for drinking yet. I was sceptical about the claims it was diastatic, it having been roasted to EBC 30; I was completely wrong, the brewhouse efficiency was 78% (I normally count on 75%). The beer is close on 7% ABV which perhaps I wasn't expecting.
As for Amber Malt, that can be very variable: Simpson's creates a wonderful mild grainy/coffee (?) flavour, where as Warminster's was distinctly roasty. Crisp's I guess will be more like Simpson's, Thomas Fawcett's more like Warminster's?