Update from the dealer; the jolt is worn engine mounts - they're going to try and have them replaced today (presume under warranty as payment wasn't mentioned). They pressure-tested the coolant system and there doesn't appear to be any leaks or evidence of one such as coolant on the undertrays etc. The service advisor guy said these engines are known to use a little though, and if it keeps rinsing through it every few weeks to book it in again and they'll re-investigate.
Brakes and pads are also fine they said; perhaps it just needs some decent stops from a slightly higher speed to give them a clean or get some heat into them as I'm really only trundling to and from work in a 30mph limit (and usually achieving an average of 10-15mph in rush-hour traffic).
All very reasonable really and I'm glad I wasn't just imagining the start-up jolt.
Edit: they need to keep it into tomorrow - apparently it's a 5-hour job...!
