Volkswagen Polo glow plug pre-heat + sticking/stalling starter motor

This is my first diesel engined car so i am feeling my way a bit.
During the current cold weather i have had difficulty getting the engine to fire up first thing in the morning partly due to the starter motor stalling for 2/3 seconds at a time before it finally turns the engine fast enough for it to fire up, and is ok for the rest of the day.
Also i found that although the glow plug symbal on the dash lit there was no volts getting to the glow plugs.
As the car is under warranty i got the garage to look at the problem.
They diagnosed a faulty glow plug relay and changed it, giving me the supposed faulty one - which i tested at home and found it to be ok.But still no volts getting to the glow plugs.
As the weather got progresively colder so the engine become more difficult to start first thing - due to the stalling starter motor but was/is no problem for the rest of the day.
Eventually i started measureing the battery voltage during startup first thing in the morning and found that it was being draged down to about 5 volts during the period that the starter motor was stalled - and then a fault indicator lit on the dash.So i took the car back to the garage.
This time they diagnosed a faulty battery - in spite of the fact that the battery fitted was only 2 months old. I must admit though that starting from cold was easier. Also at this point i started to notice that the glow plug relay operated when starting from cold first thing in the morning but not after the engine has warmed up after running for a few minutes.Also i have found that keeping the battery fully charged over night makes starting a lot easier.

Now for a tech. question - does any one know under what conditions should the glow plugs be pre-heated. Is it under all starting conditions or under very cold conditions only.
Also do all polo starter motors tend to stall under cold start conditions

thanks a lot Rodney