Would it make sense to book one ways, flying on the day or in the direction that the train is less reliable or more expensive?

I use my DC miles on domestic flights a fair bit, but then I get a free Gold upgrade to Business Class which is of course not available between Heathrow and Manchester. I also only tend to use them on Fridays/Sunday when the revenue fare is usually about £160 per person whereas your fare is half that.

Your Air Miles idea is good, although personally I think the fares as so low that cash is the way forward.

Personally I keep my Nectar points for Flybe bookings as their fares tend to be extorionate on the routes I need (EDI/NQY and EDI/SYY) and I resent giving them any more money than is necessary. Same goes for Easyjet, especially with Easyjet's lower taxes...Nectar points can be really useful in saving cash that would otherwise be wasted (no FFP).