Out of interest, the 5 transformer restriction is Mk being cautious. The reason they suggest this is that the very cheapest transformers use poor quality capacitors. This will cause a "capacitance ringing" problem when they are ganged up (more than 5). This causes the coil in a dimmer (all...
Another point about GU10 Mains Halogen lamps is that when they "blow" a large plasma is created which, unfortunately, will destroy the triac of a leading edge dimmer. Better quality branded lamps are protected against this phenomenon. I would suggest that you replace the lamps with brand name...