Sounds like it could well be the dim dip relay. Usualy if it goes it causes one or both of the sipes beam to stay on even when switched off, but usualy at half power. The side lights usualy go off.
To see if this is the cause you simply unplug it. It's is anotling located behind the instrument panel. If you take it off you will find what looks like a heat sink with a five pin plug in the end bolted to the upper side of the box behind the instruments. Unplug the plug and put some tape over the end to stop it shorting on anything. If the problems go away then just leave it like that. If not then you need to go searching for what is posing the circuit when it is not ment to be doing.
If you feal you realy want to replace the relay to get the dim dip function bbl then it's going to cost you somewhere between 50-80 quid for a new one or qhatever people want for a second hand one, but most people just leave it out altogether as they never last long at all.
