If it's the size of a rat, and shy with it, it's almost certainly a rat.
And where there's one...

Depending on your preference, poison, traps or the next door neighbours jack
russel - although I'd not go for the latter because any vets bills for bites
to the dog could cost you a lot!

You must do something and I believe rats are notifiable (you have to tell
the council) but providing you're taking steps, they almost certainly won't
care. And you won't need me to warn you that rat poison should be handled
with care as anything else that eats it dies too!

Do you get them a lot? If so, you might want to invest on one of these
tought, but expensive, rat poison boxes. About £90.00 I'm told but
obviously handy to have as you can place it where you want, filled with
poison, and it's relatively safe.

Paul DS.