Clare I am gutted for you
We had rats two years ago. They were under my Margie shed! Vile things. I have lived here over thirty years & never seen one before. Next door got a rat man out who said it was due to council cuts. They had stopped baiting & trapping the sewers & this had caused an explosion in numbers.
We all were very careful about not leaving any possible food sources around & I got some lockable bait boxes & put them under conifers on two sides of my garden where the rats were running (I put up fencing to ensure the torts had no access)
I have since stuffed the underside of shed with rat poison blocks & stuffed any gaps with wire wool (wilkos, cheap) I believe they also hate peppermint, so mint teabags, essential oil.
It worries me that I think I heard that rats like to return to where they were born, don't know-how true this is?