Test fitted the cab today and came across a hiccup, but managed to resolve it.
There are two support arms going from the cab floor to the chassis - they couldn't get around the engine! I searched backwards and forwards through the instructions to make sure I'd put everything in the correct place, then came across a small inset diagram. It showed the two support arms I mention but they were a completely different shape. I'm not sure if that's how they were supposed to look or if the diagram was trying to indicate that they should be modified that way but it certainly didn't say so or have the cut/trim icon that appears in other places.
Anyway, I figured that was the only way to get the cab to fit so set about them with a dremel. Here's the slimmed down version on the left compared to the original
It's pretty flimsy now as it's less than half the thickness it was.
And here's a poor shot of them in place. You can see how they now fit around the engine, whereas before they just splayed out around it distorting the cab floor.