Hi,
I fixed this a while ago but haven't had chance to update. It turned out this had had a previous repair on cylinder one due to impact damage to from a foreign body entering the cylinder. I stripped this down to gain better access and reamed out the glow plug hole and once again fitted a new glow plug and it cured the problem. The only conclusion I can draw from this is that there must have been damage around the glow plug causing it to distort when fully tightened (didn't feel tight going in). Thanks for the advice given.