Head Lights flickering

Travelling back from Mr. BigApe's bbq I noticed a very disturbing fact that my headlights were flickering.
The best way to describe it is comparing it to old & new batteries on a torch light, I was basically riding around with "old batteries" style lights.

So onto the facts:
- What ever it is, it also affects the dash lights. Trying to read your speed on a dim lights ain't very fun at night.
- It is intermittent. There were points where the bike got a bit of a knock from the surface of the road where the lights would get their full strength back
- I can't determine if it is speed independant. I don't believe so. After stopping for the roundabout at J14 on the M25, I didn't have lights at full power. However, after stopping on front of my garage I had them at full strength
- I didn't try putting on the full beam while lights were at half strength, I was too worried about the electrics cutting out on me while on the M4. I just decided to nurse the bike home.
- At some points my indicator lights were nearly as strong as my main beam.
- I don't think it is "dodgy" light bulb. It affects the whole light system and not just one light. Besides, fillaments don't behave like that.

So, this leaves me wondering if it is either a chafed cable or a bad connector. Has anybody experienced the problem? Anyone got any tips for fault finding this particularly nasty gremling?

Cheers. Raf.