In reply to Jim C: I think it's very unlikely that mortgage rates will fall, even if base rate does (other than trackers, which have to) - firstly a lender's cost of funds is affected more by what they have to pay on savings, and that's already about as low as it can go, and also secondly the other factor, in the case of a fixed deal, is the swap rate they can get - which will factor in expected future movements. These have already fallen in expectation of a longer period of low rates - there was an expectation that rates would start to rise in 2015, but that's now receded - I've been a "2016 man" for a couple of years, as I always felt the fundamentals for rapid growth (and therefore inflation) still weren't there, as the effects of government cuts hadn't fed through to real life.
I'd fix as low as you can for as long as you can, assuming you're going to stay in the market - you can always port your deal to a new place.