What's the collective wisdom on this?

I'm losing coolant very slowly. Can't see any external leaks, but it's difficult to see all the pipe connections. There's no smell of leaked hot coolant, so I think it's water-tight. I had the gearbox replaced 600 miles back, and the garage that did the job had disconnected the two main hoses to lower the subframe. They'd refilled and bled the system, and run the engine for some time with everything seeming normal. When I collected the car, I went for a short drive and not far up the road it overheated. I gingerly drove a couple of miles home and put a kettle of boiling water in as quick as I could. Once it had cooled I topped up with OAT. It turned out that they'd missed the bleed point on the heater - it was the only one to pass air when I re-bled it myself.

My concern is that there could now be a weakness in the head gasket (replaced as a precaution about 15000 miles back during some other work). There are no other symptoms of HGF - the dipstick is clear for instance. I'm checking the level every time I go anywhere, but haven't gone as far as getting a Lowlarm. So does this sort of slow loss progress to an actual failure, or will it just continue?

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