At that age I personally think, walks are not important, in fact none of mine have ever gone for walks. You have old ferrets, I would try to keep them as rehoming them at this age would be very unfair on them, its akin to homing a dog of 12yrs or so. You have obviously looked after them beautifully up to yet, please try to keep them in their twilight years.
Dawn.

Hope you get something sorted Nat but your neighbours sound like a couple of control freaks to me, if the ferrets go what will be next? if the garden is communal that means it's there for all of you to enjoy, not just them to keep in pristine condition. Does it say anywhere in your lease that you can't keep pets or use the garden to house your pets, if not then I would very firmly but politely tell them to stop interfering. Even nice people can become overbearing if they keep getting their own way all the time, you have rights too you know.

I think your neighbours are mean too,hope you sort something.
Dawn.

Go with you heart.

Hugs

D xxxxx