They came out and checked on Friday, no leaks.

Since I was paying for an hour anyway I had him recommission the unit as @Aredwood recommended, fix a small leak in the toilet (he was a gas-fitter and plumber), and got some advise about replacing our shared water main.

He did advise me that you use a more gas by stopping and starting the water unit, e.e if you use the hot water for 3 short 1 minute bursts in the bathroom in a ten minute period you are better to not stop the hot water in between, but instead slow the flow right down so the unit doesn't turn off. Or if 2 people are shower one right after the other, better to not turn the shower off in between. How much we will remember to do that in practice I don't know.

So even though there was no leak to be fixed, it wasn't a wasted trip.