New cat hiding

Hi all

After several years without a cat, I adopted a lovely black female on
friday. She's about two/three years old. When her owners brought her she
was quite assertive, checking out the new flat and wandering around. Even
let me feed and stroke her etc.
A few minutes after her last owners left, she hid under the sofa and has
pretty much remained there. She hides under the bed at night when I am
there, but the sofa is her favourite place.
She lets me stroke her, and enjoys it when I do, but my arm is only so
long!
I can feed her by giving her biscuits where she is and last night, while I
was asleep she came out and ate, and used the litter tray.

Around half past midnight last night she came out from beneath the sofa and
lay down next to me and rolled over, allowing me to stroke her. But a
neighbour came home and banged their door so she hid again.

She followed me into the bedroom when I went to sleep (but didn't stay), so
it's not as though she doesn't like me - just seems very, very, shy of her
new environment.

I've never had this problem with a cat before - usually they take a couple
of hours to get used to the new environment.

Is she okay, do you think? I'm worried about her becoming dehydrated - she
doesn't appear to have drunk any water (though in my experience cats need
to be shown there is water there or they miss it!). How long do you think I
should leave it before I get worried?

Should I avoid having the TV and radio on, or should she be allowed to get
used to the noise of her new home?
She used to live with another cat (older and not related) but he was "in
charge" in her old house. Her owners thought her behaviour when she was
first let out here suggested she was being intimidated by the other cat and
she would be more assertive on her own.

TIA

Jonathan (UK)