Just thought I'd add a quick comment in case anyone else has troubles in the cold...
I live in Norway and so starting in -12 is often an issue! Therefore I use the cold start lever quite a lot
Over here people buy diesel warmers as add on's for the really cold weather (-20) but shouldn't be needed in UK! I have had the plugs on mine tested and they are okay, but sometimes it needs the cold start lever pulled out and then after the glow plug light goes out turn off the ignition and repeat until the light goes out. I do this 3 or 4 times quite often just to be sure and generally try without the cold start lever first. It then starts no prob's.
In the gararge at work I try to avoid the cold start lever as it causes a small 'smoke bomb' on first start

Therefore, I prefer the 3-4 (or 6-7 if really cold) glow plugs light up option. I guess the plugs wear out as they get older anyhow as said above.
Just a thought for anyone reading still!