Looks like I've got a slight coolant leak. The level in the expansion tank has dropped from max to min over the course of 4 months and about 4k miles, so not exactly a gaping hole. I walked to the little Ford garage near my work to buy a bottle of coolant so I could top it up and monitor it more closely, and they said "just go get the car and we'll check it out for you and top it up". After they had a bit of a poke around, where couldn't see for sure, they topped it up and put some UV dye in and I'll be taking it back next week at which point any leaks should be visible.
Fingers crossed it's nothing too serious or expensive, but I can't imagine it being too costly. We shall see.