We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember "ironer"
saying something like:

I'd like to fit an operating temperature indicator to the oil boiler.
At the moment a mercury lab thermometer is puttied onto the outgoing
(flow) pipe. It was one of those quick temporary fixes which became
too permanent & it's now time to recover the said thermo.

Any suggestions for a permanent replacement please? A simple and
cheap-ish solution would be ideal. Accuracy takes second place to
readinngs being consistent as I could calibrate against the mercury
thermo. An ideal solution would be a digital display showing both flow
& return temperatures.

There's plenty of small cheap digital multimeters with a temp sensor
input - just connect a type K thermocouple to the pipe. If you want
something mains powered for constant readouts, look on ebay.

If it's only for a quick look from time to time, you could get a dual
input digi thermometer (not that cheap) or simply use two cheap DVMs.
--

Dave