I have tested the ballasts by trying a bulb that starts in other ballasts, but not in the faile ones.  I have also tried inside the house.  I have had 5 lights woking for the last few weeks even during the cold weather so I don't think it is temperature related.  I have got a new set of bulbs and ballasts so will replace them all and try again.  The only thing I will do is check the transient voltage during switch-on.

Thanks for all your help.

I spoke to a guy today, who owns a shop that does Eco lighting (CFLs and LEDs). He said he would not recommend using standard CFL fittings outside.
They sell "external" fittings that keep the lamps ticking over at about 10% and then when the PIR/microwave picks up movement they ramp up to 100% brightness (they call it Lux Up technology). I guess the theory is that they are not starting up from cold each time and they keep themselves warmish.

They do a range of 18W pl-t CFL downlighters for internal use that can be dimmed (not a standard dimmer but uses standard wiring), up to 10 fittings on 1 dimmer switch. Which is a nice alternative to halogen spots.
www.greenlighting.co.uk

I replaced all the ballasts (with the same make) and the bulbs.  No failures yet!

Well everything is still working OK.  Not sure what the cause was, but the only thing I can think of is the electrician didn't disconect the lights and did a resistance test.  When I spoke to him he assured me that he didn't do anything which could have caused it.  Alternatively there could have been a faulty ballast causing the problem.  Eitehr way so far so good!