I have a light fitting with 3x100 watt spotlights on a dimmer switch and all worked well. I have picked up power from this light fitting and added another wall light (2x40 watt bulbs). Now, both sets of lights work, but the dimmer does not. The lights are either on or off when I turn the dimmer switch. I am a bit concerned that this is not right and may overheat.
Any help?

You now have a total load of 380 watts, if I understand you correctly, which should be fine, since the lowest wattage dimmer I know of is 600 watts.

Sounds as if the dimmer is now defective, and needs to be replaced.