Did what I thought was prelimary fault-find on engine failing to warm up, fully expecting to be ordering a new stat at the end of it (TD5 disco - needle stays at 1/3, rarely getting to 1/2 like it's suppsed to)
Checked ECT sensor resitance with multimeter, all within spec. Checked for continuity and shorts between ECU and sensor, all good. Oh well, must be thetmostat then, but then on the next run she warms up to 1/2 within 5 miles, just like old times.
Conclusion: must've been a little bit of resistance in one or the other of the loom connections that got fixed by unplug and reconnect. Either that or failing wiring - we shall see