Will only stay running on revs over 2000rpm if I take foot of peddle it cuts out also white smoke from exhaust

Carry out a compression check on all cylinders to see if thats shows a fault. If its nil or low, check valve clearances and then head gasket.

Blown head gasket..

sounds like you have a blown head gasket.

Hi
White smoke is usually a good sign of un-burnt diesel so reading your post and all the things you have already changed ie the glow plugs I would have the injectors tested to make sure they are atomising properly as it sounds to me like when your car isn't under load ie at low revs you have a cylinder that isn't firing correctly
Hope this helps Nick

Hi,
White smoke usually indicates a blown manifold gasket or head gasket. First, check for coolant in oil. When you pull the dipstick, is the level too high or have a foamy appearance? If so, blown head gasket. If not, spray starting fluid around intake manifold outer mating surface with engine running. ( not near throttle body or air intake) If motor revs up during procedure, locate that area you are spraying and duplicate the motor rev up. If consistent, blown intake manifold gasket. Its always good to have codes checked first if you have a check engine light illuminated....Mike

If it is blowing thick white smoke that is coolant. You can lose a head gasket in places where only coolant passages go through so you don't always get oil/coolant mixing when this happens. Take the radiator cap off and see if air bubbles through it. You can also try pulling the spark plugs to see if one or more looks really clean.

sounds like a restricted exhaust
quite possibly the cat
disconnected the exhaust and retest

Its the problem of oil.The oil is getting burned out. The vehicle is either burning oil or it is moisture in the gas chamber if you notice white smoke from the exhaust. This is a common possibility.but in some cases this white smoke is not due to oil burning.In that case   White smoke from the exhaust is a symptom of coolant entering the combustion chamber. Normally this is a sign of a blown head gasket or cracked head. And  this is not a common problem must be repaired immediately. You must STOP driving this vehicle or you will cause serious engine damage. you will have to get both this possibilities checked.
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Check for vacuum/intake leaks.

Do It Yourself Diagnosis and Repair

Sounds like a MAF sensor problem, clean it 1st, if no change replace it.