Fiesta MK4 1.25 Cold Idle Problem

This seems a common problem with the Fiesta 1.25 MK4 (1999) but while I've seen many posts describing the problem I've never seen anyone to say they've found the cure.

Car starts fine, idles about 1100rpm. For the first half mile the car drives fine. Then at next junction, when you dip the clutch the engine either cuts out or idles so low the alternator light comes on. After about another half mile the engine is beginning to heat up and all is well for the rest of the trip. There are no other problems with the engine - lots of power and very economical. It has only covered 22k miles.

So far I've:

• Put in a new thermostat, when I bought the car the thermostat was stuck open and I suspect it was like that a long time. It now reaches full temperature.
• New coil and ignition leads
• Cleaned the idle air control valve - looked spotless anyway.
• Cleaned the TMAP sensor
• Took off the throttle position sensor but not much can be done with this from a cleaning point of view
• With an OBD reader attached I monitored the engine coolant temperature sensor, the throttle position sensor and the incoming air temperature from cold start to engine reaching full temperature and all looks normal.
• All the little rubber vacuum hoses are intact.

So, now am thinking of changing a few sensors. I believe the lambda sensor shouldn't cause a problem at cold idling but can affect hot idling so am ruling that out?

I think it's either the throttle position sensor (TPS) or the engine coolant temperature sensor (ECT) - even though the OBD reader indicates they seem to be fine. Has anyone bought these from ebay and would they recommend buying from there? They are a lot cheaper than main dealer parts or motor factor parts. Would they be good enough to at least prove the fault?

One other thing - there is not engine management light (MIL) on the dash. It doesn't even come on when the ignition is switched on initially. There is no mention in the warning lights section of the user manual of one. Did these cars not have a MIL light?