Section ummmmm 5 and 6 mastered successfully - this is one Cubase project with 2 sections. No Kontakt therefore no problems, sparkled everything up to be nice and shiny and it seems OK. The addition of small amounts of reverb does wonders.
Section 7, quite a lot of work went into it, just fussing around with the sounds, getting them fitting right. I wasn't overall 100% sure I'd finished but time was ticking on so I decided it was time to stop, export out a mixdown and then come back and check to see if it's OK.
On mixdown it crashed.
So I left it.
It does have Kontakt in, but not the offending reverb. What give? So I sat down this evening fully prepared to try all sorts of tactics, but I started with the easiest - a "save as" and then a file copy. I loaded up, pressed the button to mixdown and told it to crash.
So therefore a) it's not just the combination of Kontakt and that reverb that causes crashes and b)swapping the reverb isn't the only solution. Basically it just keeps me guessing!