Steve, I think Zulu's roof fitting is a cab vent, not for horns, can't check....as you have my books now LOL
The primary difference, apart from all that Steve mention's, between the 42 and 43 is internal. Those built at Swindon being Class 42 with Maybach engines and Mekydro transmissions and those built by NBL, which are fitted with MAN engines and Voith transmissions being Class 43.
Strangely the Class 43 are the lower powered ones by 70HP but being as the higher power ones were built first they took the Class 42 bracket, when the NBL batch arrived later they should technically have gone to Class 41 but that was already taken by the original Warships so they were moved out of position to Class 43. On the road there was no distinction between the power classes so it was only a paperwork issue.
I have only three diesels on my must have list, Deltic, Class 40 and Warship, the Heljan looks very nice but at that price I think I'd rather go for a Fred Phipps one or wait for the JLTRT one if and when they get round to it.