It seems that there is a presumption that if there is an accident
involving a cyclist, it is always the motorists's fault. IMO it
should be the other way round if the cyclist is riding a bike after
dark with no lights and is involved in an accident - after all, any
cyclist in that situation is breaking the law, so why should the
driver automatically get the blame?

I'm not aware of any presumption of blame in any RTA.
No, but if the worst happens, just wait and see what the attitude is.

And I cannot understand why the law allows the sale of cycles which
are not fitted with lights. It won't allow any other vehicle to be
sold without them for use on the public roads - why should cycles be
any different?

Probably because it won't make any difference to the problem. Just because
a
bike is sold with lights doesn't ensure that they'll be replaced after
being
removed to prevent theft, or that the batteries will be replaced when
exhausted. They're not really like car lights at all...
No, but people could not use the excuse of 'this bike doesn't have any
lights'. And as for theft - you can get bikes with the lights built into the
handlebars, not just attached, and you can get them with dynamos to power
the lights - mine has, so I have never needed to buy a battery for the bike
lights.

Anne