Just another thought - If I were to call BA and tell them that I no longer need the LGW-AMS leg (or instead book it a few months later) and instead need to make it to NCL pronto that morning, would they need to reprice my ticket? I've paid full fare WT+ and then u/g to Club.

I know what you're saying Swiss Tony - I figure that even if the inbound flight from Houston is 20 mins late, I'm going to miss my NCL flight and since I was planning to book it as a cheap £70 ticket, I can't see them helping if something goes wrong (I'm still only a Blue). I suppose I could book a full-fare one way from LGW but that's nigh on 200 quid and my employer won't refund me.