At last!
New clocks and 2 new sensors (all used) has it working! 1st replacement sensor worked initially, but died very quickly. Suspect the fault on the original clocks killed it.
Replacement thermostat housing and 'stat fitted and she seems like she's good to go.
I'm sure something else will rear it's ugly head in the very near future. It needs replacement chain/sprox, but not counting this as a fault as they are consumables.