Starting troubles

I have read through a few posts on this topic, and im still a little unsure where to start looking for issues with this. I was having trouble for a few days a couple of months ago where the starter would turn the engine a little longer than normal before it fired. Over about a week, this got gradually worse, until it didnt start at all. However, it started straight away every time after being stood all day at work (9hrs). I tried jump starting, and it fired straight up after being stood fopr two days. I bought a new battery, did a service, (fuel/oil/air filters, oil ect) and this seemed to sort the problem. That was a month ago. This morning, it was a little colder than normal, (9°C) and it struggled to start. I have bought some glow plugs, which I will try but I thought that they only worked in very cold conditions anyway? Does anybody have any ideas what I can check or what may be causing the issue?

My thoughts are - Starter motor works fine with new battery and when jump starting, so unlikely
- Its not that cold so glow plugs wont make much difference
- fuel pump - last resort to change, although know this model has fuel and oil pump issues!
- Could it be a short that the damp/cold effects?

This is for a 2006 2.0TDI 170 Any advice appreciated, will also check the voltage from alternator tonight. Sorry for the long post!

It seems after I fitted the new battery, changed the glow plugs, and gave it a service, which was due anyway. The problem seemed to go away, put a meter across the alternator and this all looked fine. I now have trouble with the car going into limp mode, seems its been trouble ever sine I bought the thing!