Starter motor sluggish.

[rodders 204]

Morning all, I'm hoping for advice please.
My model is 1.9TDI 130 highline (6 speed standard) 2004. 108,000(k) only.
I have had it for just a year, and have had quite a few problems to put right since the last owner neglected it.
I had fitted as a matter of course new timing belt and full belts, pump kit. So getting there. The latest issue, is that the starter motor has become sluggish. She always fires, but seems to crank the engine noticeably slower. When this began, I had the alternator charging rate checked, this was said to be fine, so I bought a new battery. As said, new belts, so tension should be set right. I have been under there, and removed all cables/ cleaned all connections up/greased, and replaced nice and tight. I also found an earth strap finger tight near the front bottom of the engine, which was cleaned and re tightened too.
At first, she turned over much better, and I thought it was prob cured, but it's slowing down again whilst cranking after just a week. The temps here (Spain are around 13/15), so I don't think it's a cold starting issue as such. When it cranks, the glow plug light only stays on for a couple of seconds, (shorter than usual). I'm thinking that this could be the next poss reason. Can anyone shed any light on this, or give advice please...... I know glow plugs are not expensive, but just not sure if I'm barking up the wrong tree. Thanks in advance for any tips. Cheers, Rodders.