A bit of an update: I got my compression test kit yesterday and got to work today. All seems relatively OK. Getting 8.5 to 9 bar (125 to 135 psi) on all 4 pistons versus a wear limit of 8 bar for a DG engine according to the Haynes manual. I was expecting much worse. Would these kinds of compression results warrant a change of piston seals normally?
Also, no smoke on start-up today or last night but had some after a minute or so on restart just now. Really seems to be intermittent. My latest theory is that the coolant leak might be just affecting one cylinder and if the engine stops with that piston in the top position, you don't get much water ingress. Also got my tacho but still waiting for the oil pressure test kit I ordered.
Still in two minds whether to take on a stripdown. I guess I'll see what the oil pressure looks like first. That will determine how far I go. Sorely tempted to just do a set of head seals but guess I should do the piston and oil seals also...