We have a balcony (back of property) and we have 6 direct neighbours all of whom have a balcony too.

You can only access your balcony from your property.

Past two nights we've had a little cat come visit us and it's sending my Amber beserk.

I have no idea where cat is coming from. It's not neighbour either side and from what I can see all the other properties are closed / lights off etc.. BUT cat could only access from the other balconies. There is no street level access / public access.

Amber goes out everyday when it isn't wet and she has a climb on the rooftops and a little wander through the neighbours balconies, she is known and not a pain (my neighbour has canaries and often finds Amber inside his house but he doesn't mind!)

Amber was so distressed last night she sat and hissed at other cat for 4 hours through the window.

OH came to bed about 1am and said random cat was still there but "she'd" disappeared this morning and has just returned now. Same time as last night.

It could be that someone is throwing her out same time each night? But she must have just arrived? We've never seen another cat in the 3.5 years we've lived here.

She looks quite skinny but in good condition. I've said not to feed her at all but she just sits by the balcony door...

What to do about my poor cat though? We tried shutting her out of living room (so she is away from balcony) and she just cried for ages and kept me awake.

Difficult one, do you think the cat is a stray?

Just try to ignore it and don't feed it, unless you want them turning up all the time!( I know, easier said than done!)

We had the same problem here, rang cats protection and they were good as gold, took him away and rehomed him!

If you can, put a collar on the mystery cat with a contact number or email asking if the cat is owned. If its not then contact Cats Protection to see if they can help.

It's still here

It can't be a stray as it has to have come from one of my neighbours houses? Unless cats can now scale 50ft walls??

Everyone's doors / windows are shut, so I just can't work out where it comes from.

It's obviously Professor McGonagall.

Whilst the neighbours doors and windows are shut, they could simply let the cat out and then only open the door when the cat 'asks' - we used to do this for our old cat. If the cat is new she may well just be checking out the territory - if Amber is the only other cat then naturally new cat is going to want to establish a pecking order.
Are you sure new cat is a girl? Even neutered toms take an interest in queens as its in their nature to check out queens as potential mates. Since I got Brooke I've seen a lot of cats around who I've not seen before - presumably checking her out for when she comes on heat (long wait there boys - she's been done!)

Have you tried asking neighbours other than the ones either side? Could it be going from balcony to balcony (i.e. several away)?

I've knocked on the other neighbours (and left a note for those I couldn't speak to)

I am not sure it's a lady cat, but it looks like a lady

So if both Amber and boy cat have been done the boy cat may still come and check out Amber??

Cat was here when I left but gone this afternoon.

I understand that people leave cats out but surely this is when the have a lot of space to roam?? It's a bit selfish to leave a cat out overnight when it only has access to 6 balconies - also where is it going to do it's business?

As I understand it a complete tom will try to 'claim' a queen before she's in heat - they won't know the difference between a neutered queen and a complete queen off heat. I'm fairly sure this behaviour happens with neutered toms too, though to a lesser extent.
However I think it more likely that its simply a new cat sussing out the competition - they may be able to co-exist once they finally meet or they may need to divide up the area. Hopefully the noisy face off through the glass may be enough.