is it against the law not to register on the electoral roll? and how does it work if you are a tenant in 1 flat in a building of 6 individual flats, does each tenant still have to register or is it just the owner of the building who registers?
It is against the law not to register, though the law is rarely enforced.
As for flats, it depends on whether the registration office have recorded the building as one dwelling or several. Normally each tenant has to register - and can indeed insist on registering separately if he wishes. From memory (and ICBW) there's something on the form asking if there are other occupants anyway.