Right there's something wrong, you have swapped the dimmer with one you know that works and the fault is still there.
Have you checked the dimmer serial ID against the link you posted and made sure it is a model type that is happy to work on the 200w load you have?
Have you checked the wiring at the lamps and made sure there nothing odd there?
(see diynot wiki for diagrams that cover all the types of uk lighting wiring).
Are you absolutely sure the lighting you want to dim isn't double switched, sure that there isn't another switch somewhere that allows the lighting a secondary point to switch on / off?
We can only help if you provide us with info, simply re posting "help" when you have been asked to tell us things (and not done so) is frustrating.
If you really want this solved I would suggest:-
Remove the lamp fittings on the ceiling to reveal the wiring, picture and post both.
Picture the lamps used in the fittings or give us exact detail of the type, make, model, rating.
Remove the offending dimmer, picture and post (a) the inside of the dimmer with enough detail to read the model type and the terminal markings (b) the wiring terminations on the dimmer.
Do that, and just maybe our frustration and your vagueness can be resolved.