So these houses are your typical new build estate, all squeezed on like crazy to the smallest plots imaginable with naff all parking, then sold at grossly over-inflated prices to gullible people who think they'll have no problems as they're new. (I'm not keen, can you tell??

I wouldn't say built so much, more thrown up as quickly as possible. Anyhow, the "chimneys" on the roofs are pre-fabricated timber framed boxes, clad with fake brick effect panels and with what looks like a plastic chimney pot on top, that is plonked on top of the roof trusses before the tiles go on. They serve no purpose at all - no flue of any kind goes to them. As for flashing, it's a lead free zone here - the roofing membrane passes right under the "chimney" and it looks like some kind of pre-fitted corrugated aluminium flashing is bent down over the tiles.
Back to lead though - with that very tall and slender chimney I've got on the side of my house, I'd be concerned about a DPC compromising the structural integrity - i.e. introducing a weak point in the masonry that could cause problems if a storm was trying to push the stack over. As it is now I'm not concerned, it's managed 120 years so far!