Forum:
after changing the headgasket , doing the timing and rebuilding the head i have noticed the car is occasionally emitting white smoke and there is some soot aswell.
it is using up a noticeable amount of oil but coolant level is fine any ideas 
idle can also be erratic
could it be poor valve clearances?
No, excess valve clearances
No, excess valve clearances would not produce smoke.
Any use?
White smoke at star tup is condensation.
White smoke when engine is hot is coolant leak.
Black smoke is excessive fuel burn.
Blue smoke is oil burn.
Possible problems .....
oil leaking down valve stems.
oil passing piston rings and burning in the cylinders.
Fix
Try using a thicker viscosity oil. If you use 10/40 then try 15/40. The first figure is the viscosity at cold. The second is the viscosity when hot. So if it blows at cold use a higher [first figure] viscosity.
Try an oil additive which will coat the piston rings and stop the oil leaking into the combustion chamber.
if i was keeping it
ok then, if it was a sporty i was keeping then i would rebuild cylinder block with new piston rings aswell as other bits however its up for sale and i want to fix the problem as much as possible 4 the next owner.
just put an oil additive in hopefully it should help. next would be oil , possibly 20/50
i changed stem seals during head rebuild so should be them id hope
Try both
I would go for both the additive and the thicker oil. No point in using an additive if you later change the oil, since the additive will be in the waste oil.
Perhaps use 20/50 plus an additive. I would consider 30/50 since most older cars used 30/40 as standard. Newer cars dropped to 20/x or 10/x viscosity as a economy for the fuel usage.... the less friction the less fuel was burned. Problem is the lower the viscosity the more like water becomes the oil.
I suggest you go to your local DIY motor shop and see what they have on offer in the form of oil viscosity.