A problem with an "unwanted" cat

Hi all Really wasn't sure where to post this so feel free to move. Since we moved here last month there has been a cat coming into our garden, the cat is very similar looking to Harvey our new addition whom we "inherited" from the last tenant of the house. I have found out it is Harveys father from neighbours and it belongs to a lady round the back of our house. Well, she clearly does not really want the cat as it is out all weathers, its been hammering it down with rain the past few days and it keeps coming in our garden wet through. It doesn't look underfed though so she must feed him.
The problem is whenever our cats go in the garden (supervised) it comes and attacks them. I keep shooing it away and it keeps coming back, our cats haven't been out the past few days because of the weather but they look out of the windows, see it and growl. It really is upsetting my kitties and I have heard from neihbours that it is a right menace.
Trouble is, I feel sorry for the poor thing, is it the cats fault that it is left out all the time and just wants a bit of TLC? My Harvey was unwanted by the *** that lived here before and the change in him since we got him is amazing, his behaviour is excellent now and I am awaiting a voucher to arrive so I can get him neutured for low cost wich I know when he is fixed he will feel even more loved and happy.
My hubby moans at me as I want to take pity on this poor kitty when he is just terrorizing our cats but I can't help it, I cannot take him on, I estimate he is 7-9 years old. I am not sure what to do, I have considered phoning the Cats protection league and getting them to take him or have a word with his owner but last time I phoned them about a cat they were that full they weren't interested. The problem is most of the shelters over here are kill shelters and this kitty doesn't deserve this and ultimately he is somebodys cat.
I do not want to approach his owner as I have heard she is a bit "rough" and would tell me where to go anyway.Any advice would be greatly appreciated.xx

Hun i really think you should phone them if they wont help phone the rspca they are really good ive used them a couple times in various areas of the country to, you got to remember its attacking your cats and you need to protect them first

this poor cat needs help, the rspca might just rescue it and find it a foster home.

(0300 1234 999 just incase)

Originally Posted by -_aj_-

Hun i really think you should phone them if they wont help phone the rspca they are really good ive used them a couple times in various areas of the country to, you got to remember its attacking your cats and you need to protect them first

this poor cat needs help, the rspca might just rescue it and find it a foster home.

(0300 1234 999 just incase)

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Thanks hun, I'll give them a ring tomorrow, my cats have been confined to the house the past few days, they mostly just come out in the garden when we go out there but the fact is it will still go for them when we are around. We just got rid of one set of idiot neighbours and I am not willing to gain some more, I know the RSPCA will let me be confidential. I just don't want it to be PTS as it probably just does want love like Harvey needed.Hubby says its not my problm and I have to stop falling for every waif and stray (thats how I ended up with him though ) x

dont let hubby hear you say that lmao

i hope you get it sorted though last thing you want is to have unexpected vet bills incase that cat goes for yours you dont know if it has anything - i know that sounds awful but you never know