Well, a struct eng reckons it's shrinkage, been there for donkeys years, and a result of the horizontal length of brickwork and the fact that concrete mortar has been used. Nothing to worry about, and he pointed out that while the upper part of the crack looked widest, this wasn't actually the case, it was just the mortar weathering, and it was hairline and petered out before getting to the cill, so not bits of wall rotating. Also said the brick courses weren't bowing or anything. There's another very small crack in the upper run of brickwork, and he diagnoses same thing - the windows have allowed the shrinkage without cracks but the horizontal stretch both above and below it have cracked mortar.
Repoint and it will be fine, was the advice - there's a very small area of repointing in the corner by the porch/upper bedroom already. He picked up on the DPC not being as high above the ground level as it should be (already knew this wasn't ideal but all properties in the road are identical). He says it's built on limestone not clay, and there's no subsidence in the area.
So that's all been very reassuring. He gave the property a thorough look over, and doesn't think there's anything that'll come back to haunt me. He's also willing (for a small extra fee) to put all this in writing if needed - in case the HBR raises the issue.
Between myself and my dad and the struct eng, we found a few other things that really need putting right eventually, so I'm hopeful I have a good grasp of how much I need to outlay both financially and in my own hard graft and time - I'm looking at this as a fairly long-term project, and I think the stuff is within my means to get done slowly but steadily, which is great to know.
So thanks everyone, I wouldn't have thought to go ask a structural engineer if they did a verbal thing like this - I would have assumed I needed a structural surveyor, too, rather than an engineer.
I think an offer may well be going in on the place soon :-)