Changed the glowplugs, now starts perfectly.
Measured the resistance of the new plugs and was about 0.85 Ohms - the ones I took out measured 0.5, 16.5, 43.5 and open circuit(!) - No wonder it didn't start very easily!!
The info below is given for other people's benefit.
To change them you require a 7mm spanner or socket to remove the electrical connection and a 10mm deep socket to remove the plug. Be careful not to loose the elctrical connection nut when removing and installing it, other than that, very, very easy.
For those of you that are interested: Each new plug measured around 0.85 Ohms and was rated at 11V therefore, using ohms law, the current drawn by each plug should be
11/0.85=12.9A (I=V/R) or ~52A for all four. (Similar values to what Carl was getting in his original reply) The originals are Beru, I got Wellman
formatting link
ones off eBay for about £25 including P&P.
Thanks to everyone who took the time to reply.