Help please ! - temporarily adopted cat

I've just taken on responsibility for a ginger tom cat for a couple of
months that would otherwise have been put to sleep. He's name's Barney
- URL:http://mysite.wanadoo-members.co.uk/JohnsStuff/Barney1.jpg.
It's a weird situation and I don't have any experience of looking
after animals, so I'd appreciate any thoughts or comments on how to
handle it.

The cat had scratched the owner (my landlord - I live in the grounds
of her house) and infected her with an unknown poison. She's 80 years
old and has been in hospital for 2 weeks as a result. She,
understandably, doesn't want anything to do with the it anymore when
she comes home today. Her daughter (who originally rescued it as a
kitten) is buying a new house and will take it when she moves in, in a
couple of months. She currently lives near a road and feels that it
can't live there as it's not used to roads.

I've always got on well with the cat; he's always wandered in and out
of my house and I feed him when his owner goes on holiday. As I didn't
like the thought of an otherwise healthy cat being put to sleep, I
offered to look after it for the daughter and she's really grateful.

The problem I see is keeping the cat away from my landlady. It's got
plenty of space - about 4 acres, half woodland half garden but it
likes company and usually follows her about when she's in the garden.
It occasionally plays games with her, hunting her through the
undergrowth and pouncing. I think it's bored out of it's mind and does
this to entertain itself.

For the past 2 weeks it has spent much of its time with me and I feed
it at lunchtime - the daughter has been feeding it in the morning and
evening. For the first time, for 4 nights out of the last 5 it has
slept with me and has started bringing me the occasional gift of a
dead mouse/vole. The daughter has said that she normally finds dead
mice in the house, but has noticed that he isn't bringing them in
there anymore.

Does this mean that he's accepted me as his new owner ?

The cat flap on his old house has now been blocked up and I have all
his bowls, food brushes, flea treatment etc

Is it possible/fair to attempt to keep him inside all day ?

He seems to spend most of the time with me (I work from home) and the
disappear out in the evening until about midnight. It's a bit of a
pain if I want to go to bed as there's no cat-flap. Do you think I
should get one ?

I would like to help the daughter as much as possible, as she's good
with animals but has had a bad run with her other pets - one dog dying
from cancer, another dog losing the use of it's leg because of
pressure on the spine.

Any thoughts would be great, thanks for any help !

John