What brand is the downlights, and is the HF gear built in or attached separately? Do you know the brand of HF gear, is it definatly HF type?

Is the shed insulated, what is the ambient temperature like?

Do you know if the HF gear is pre-heat or cold-start?

Fluorescent lamps in general don't like it cold, in fact they operate at best luminous flux (light output) in a very narrow range of temperature. But certainly they don’t like it too cold, the cold will shorten their life, and switching them on and off repeatedly will shorten their life even further.

Always run your fluorescent for a minimum of 20-30mins at a time.

I presume all the wiring is correct and there are no issues?

Pehaps check the earthing. I recall reading somewhere that good earthing of the metal parts of the fitting is required or higher voltages are need to get the tubes to strike.

This is a long shot but could you try splitting the circuit into 2 so that you only have 5 on each? It may be inrush currents etc destroying them prematurely. I have also been told CFLs don't like the cold but you say they should be good to -15C so I guess that cancels that one out...

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It's unlikely, ballasts such as them are designed for working in reasonable numbers, certainly 10 should not be an issue. However one should not mix electromagnetic ballasts and HF type ballasts as the inrush from the former can damage the latter. But if it's all HF there shouldn't be a problem.

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VS gear is of reasonable quality, but I don't think compareable to the quality of philips "performer" range. VS is normally put into cheap fittings in my experience. Tridonic is a brand all of it's own.

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You will do, in fact you can run many ballasts between about 198V and 250V, they have some superior electronics in them that's for sure. Very stable against mains fluctuations normally.

It sounds likely that condensation could be the problem, as you state. Maybe there are a few duff ones, it can happen with anything, but dissapointing to have more than one fail.

Rob, when the lamps go, do they appear blackened on removal? or do they glow dull purple when in the fitting?

I think 18W 4-Pin has it's own set cap of G24q-2.

From information here, and for reference (4-Pin)

10 & 13W are G24q-1
18W is G24q-2
and 26W is G24q-3

2-Pin variants have different cap & pin configuration (obviously), and
the higher wattages are a different shape altogether.

It's one of them problems where you wish you could be there to see it for yourself and make a judgement!

There's quite a large range of pin-based CFL's so it's not surprising.

Strange phenomena’s.. I reckon you've got aliens coming down in the surrounding fields at night and blasting them with EM radiation  

your all wrong, its the bloody horses lossening the lamps in the fittings!!!

(sorry)