Thanks Laughing Squid.
I keep telling myself that.
I started getting a small hiss out of the steam boiler. After pulling the lid and inspecting it, it appeared that the seal around the temperature sensor had gone. It was due for a descaling, so I pulled the vac breaker and the drain plug and flushed it with descaler (2 tablespoons citric acid power to 1L of water) a few times. I also pulled the temp probe.
Indeed, the teflon tape on the threads of the vac breaker and the temp probe were old and brittle. New teflon tape and we appear to be back in business.