**** UPDATE ****
So it went in for a check on the 19th August. The work was to replace an 'O' ring on the injectors and replace the oil. Other options were available but we started with the cheapest.
After a nice long run and about a week - things were no better. If anything the cloud of blue smoke was worse. After a week the oil level had crept up above the max mark again.
They had tried the cheaper alternative - now things could get expensive!
Booked in again early September and this time, instead of 'trying' to fix the injectors, they were sent away for proper testing. Symptoms were definitely a dripping/leaking injector.
1 injector found to be losing pressure, the other 3 OK. Fault identified and the injector was replaced (£136 for a new one + testing + labour). After running for a week - the amount of blue smoke is significantly reduced. After reading through the EGR mod sticky (and seeing some of the pictures) I thought that things might improve if the EGR and inlet manifold were cleaned out. There was a lot of black sticky goo in there.
I did that over this weekend and (fingers crossed) the smoke was nearly zero this morning.
So.... here's the symptoms and the cure - it may help others.
firstly - watch the oil level. If it's going up, then it's not oil pixies keeping you topped up, it's something getting in there. The diesel will drip from the leaking injector into the piston chamber and run through past the piston rings to the sump.
secondly - watch the smoke at start up. As the oil thins it seems to get into places it shouldn't (especially after a month or two of driving) and the extra diesel won't help either. If it gets really bad, I have heard tell that starting becomes difficult.
Thirdly - if the smoke then clears and it runs fine, then it is probably not valve guides and you have narrowed down your culprit.
Lastly - all that extra (thin) oil will get into the inlet manifold and the EGR valve. It's a mess. If you are impatient, take the bits off and clean them out otherwise, a few good runs will help.
a warning though.... if you decide to clean out the EGR and inlet manifold, cover everything within a 10 foot radius with newspaper, don't wear your best clothes, allow 3 hours and a couple of tins of Gunk, Mr Muscle etc and don't use the wife's toothbrush to get into the deeper recesses of the EGR. They don't mention that in the sticky! :doh: