HI!
White smoke is not necessarily a problem.
Yes it's true that white smoke in the exhaust can be caused by a failing Head gasket allowing coolant to be burnt - but when that happens you can normally smell the Anti Freeze being burnt in the exhaust fumes - and if it IS the head gasket then the white smoke will get worse and worse until a ludicrous amount of white smoke comes out (Particularly on start up).You'll also see an OBVIOUS drop in coolant level with no apparent leak. As things progress the engine will start to splutter as it tries to burn water and then eventually the engine will Hydro-Lock. (Which means so much the water has leaked into the cylinders that it can't be compressed and then the engine will not even turn over, you only hear the starter solenoid clicking. (If that happens then remove the plugs and turn over- if water sprays out then it's a Head Gasket).
I have found that KSeal actually DOES fix a blown head gasket if you use it correctly and you're fixing a water blown gasket not an oil blown gasket. (Mix with 2 litres of water, shake well, pour directly into the a hose leading to the block, do two or three heating / cooling cycles without going over 1000- 1500rpm . ie car idling on drive). K-Seal has glass particles that melt if they get into the combustion chamber and seal up the leak. (Fixed my 1998 E36 2.5 beautifully after a heater hose came off and it cooked the Head).
A more likely explanation for your intermittent white smoke is simply that the car is not being used regularly and I assume you live in cold climate (England?) and have a car that's standing OUTSIDE for long periods. If so then condensation builds up in the exhaust and when the car is started it turns to steam. You'll get a MASSIVE / UNBELIEVEABLE / RIDICULOUS / UNFATHOMABLE amount of steam & white smoke from the even the smallest bit of condensation (Steam Volume is x1500 times the volume or water - Or something like that). It will also take a REALLY LONG TIME - Like a week or two to completely dry out the exhaust.
I started my mums Volvo after it had been standing for 4 years - It was a Smoke machine. This is normal and is absolutely nothing to worry about. The engine temperature / heater on it were fine, no boiling / gurgling and no loss of coolant. The white smoke diminished over the following 2-3 weeks as it dried out the exhaust with use.
If you're still worried it could be the Head Gasket - Put some K Seal in (Or similar) and like DO IT NOW.