What is this animal in my loft?
I dont think birds would be so sneaky.  It is going to be a mammal.
You can catch it.

source
I like live catching over poison because poisoned animals die and rot and stink.
Once you catch one you can display it to your landlord.  Then he can pay for a pest control person to come catch the rest and most importantly, seal up the way they are coming in.  Your landlord should be enthusiastic about protecting his property.
In case it is not a rat but a squirrel, or a hedgehog or a bogle or whatever you have in the UK, you will be able to get a good look at it and plan accordingly.