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Spoke to the neighbours (in the Victorian semi-, so mirror image of ours), and they had a crushed surface water drain pipe when they moved in (~3 years ago) that had caused their bay to show cracks.
The surface water drain out front doesn't appear to be obviously damaged, but it's still on the list of suspects, pending a bit of digging. it does look as though the drain on the road may be full, so that's also something to look at. The soil on the drain side is damp though, where it is dry everywhere else.
I've cleared the some of the godawful decorative pebbles out front, to have a look if there is anything obvious there. I didn't realise that the front garden was concreted over at some point (joy of joys). But its obvious that someone has removed some of the concrete around the bay.
To add to the general fuckery, there's this nonsense: a PVC waste pipe, going through the wall (at the expense of some removed brickwork) that goes nowhere.
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One other thing it could be, also related to water, is a leaking pipe / drain causing water build up.