Hi PattiSorry - maybe I was being a bit vague

As usual

Or perhaps I'm not thinking of the same form. I get one every year and it does say that you could be fined if you don't complete it. It's possible to complete it by phone if the information you gave last year hasn't changed.
If this is the same type of form then yes, it is often called an electoral register, and yes, there IS an obligation to list those over 18 who live in your house. However, its purpose is to determine from the information you provide, the number of people elibible to pay the council tax.
Council tax, is by law, an obligatory payment since it maintains public services. Because it's a law, non-payment results in a court summons. If you don't attend the court you can either be found 'guilty' in your absence or arrested. I've known it to happen. Councils chill you with the prospect of a fine because court proceedings cost them money.