It sounds to me like you need the services of a professional pest
control man (or woman). - not very DIY I know.

I had one round last year.
- I had rats in a ceiling.

He used some blocks of poison, which I believe are only available to
professionals.
- I think the manufacturer is sorex. - http://www.sorex.com/

We managed to keep my rat out by blocking it's entry point.

Apparently rats love the blocks he put down, and if one was still
around it would definitely taken it.
- he told me he recently put 12 quite large blocks down in a farm with
a rat problem - they ate the lot.

Interestingly he used wire to fix the block to the ceiling joist, that
way the rat couldn't run off with it to it's hidy hole and it meant
that he could examine it to see how much had been taken.

Also, he told me that by law, all traps must be checked every 24
hours. A trap may not instantly kill, and may cause unneccesary
suffering.

By the way, you do want to get rid of them - for some reason they love
to chew mains wiring.

Regards,

Chris.