Hi Peter,
the wires are and casings feel brittle, quite a few have been painted over at some stage as well, I think replacing them is the sensible option. I also thought that new wires should have less resistance and with any luck may improve the headlights and prevent future electrical maladies caused by dodgy wiring/connections.
I'm going to go up to the garage later this week and will check what colour wiring I need or I may just order a mixed bag of thin wall to get started, something like
http://www.ebay.co.uk/itm/251239196538, some heatshrink may also be worth ordering ....
Chris