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I think trailing edge is generally best with the current crop of GU10. It's the lower voltage range of the dimmer that is too high if it's not specifically for led's. Given that 6x5w leds makes 30w and a dimmer designed to work with halogen would have a lower range roughly equivalent to 1 halogen bulb, so 40w or practically 50w. The x-pro dimmers are programmable so you can set a lowest and highest limit, just buy the module for £10 and fit it to the existing plate with the existing knob.
Another problem people have with bulbs blowing is caused by cheap bulbs. It's difficult to commit to buying a whole load of high quality GU10's if you've had a lot failing so you buy cheaper bulbs and they fail more regularly.
Don't mix led and halogen on the same circuit, it can upset some dimmers and cause them to fail. It happens quite a bit in tenancies where leds are supplied but the tenant wants to fit the cheapest bulb. This is less of an issue now that prices are coming down.
Try to get a bulb like Phillips master led from downlights.co.uk around £7 each, high cri and decent manufacturing quality.
I'm not sure about wiring or 'quality' of mains supply having an effect although I hear it quite a bit from potential customers. Normally when I'm looking at the systems involved they have a lot of lights and they are not constantly replacing the same lamp!
I guess that we've all heard the halogen bulbs buzzing when the filaments start to vibrate which usually means they will be giving out soon. Dimmer quality could have an effect on the filament I suppose. Wiring itself should be right or not with a variation of insulation resistance which would seem unlikely to play any part in the life of a bulb.
There are some issues that arise with the connectors in the 12v halogen variety, sometimes when you change the bulb the problem of the connection goes away for a while but the old bulb is still fine, it's just that if it's stopped working you don't test it generally.
@Dramatic_Hammer @TW @Airhead
Thanks for all your replies. They're (supposed to be) on a trailing edge dimmer which I thought were supposed to be ok with LED? Anyway I appreciate the help and gives me plenty to get on with, cheers!