Anyway, never mind. Overall I'm quite happy with how it's come out. I decided that, apart for the mortar itself, I'd leave the tank as is. Some AVRE have numerous other alterations, for attaching fad Ines, bridging equipment, etc. But some that carry the mortar are otherwise quite normal. And even amongst the mortar variants there's a great diversity. In fact even the mortars themselves vary a fair bit!
All that remains now is to paint the bugger. I'm hoping that I can go to town on this one, and make the single colour scheme really 'pop', as they say. As for markings I'll do some research, and try something different from the decals supplied with the kit. I really like those square markings you see on British tanks. If they're appropriate, I'll use them.
Just before turning in, and whilst half watching John Carpenter's Body Bags horror anthology, I undercoated the tank in Halfords grey. Tomorrow painting and markings!