Fitted an inline thermostat after draining and flushing the system with the hose. Now my heater works when I want it to instead of only when it's hot outside, and the temp gauge is rock steady.
I pushed along a fast B-road for a bit, then let her idle - just to make sure there was no overheating, but all good.
Stopped at the local garage on the way back to ask about my non-functional aircon. Any chance of taking a look next week I said? "mm.. no, but I can have a look now!"
Gas was very low, he refilled it and all seems good now, except that the rad/condenser fan isn't coming on so the A/C doesn't like standing.
Next job probably bumper off to fit a new slow speed fan resistor (after checking that is the issue).