Just tried it and 3x 42w halogen and 3x 7w LED lamps all dim perfectly.Have you tried with the 3rd dimmer yet?
So the simplest answer was correct: faulty dimmer module. I just assumed I'd cocked-up the wiring circuit, although I couldn't identify a fault. Not all is to waste though, as much has been learnt and I will apply the advice given and update the thread when it's all done.