would 110 sq ft be ok for 3, max 4 chickens?
Good. A little small, IMHO, but it'll work. Use mulch or straw in the chicken run to about 3". Keep it freshened and stirred. Replace it about twice per year.
WHat you do not want is mud. Ever.
we have a secure garden so they would get out.
I assume this means they will get out to forage the yard/garden area. Good. But mind your plants - the chickens will nip at the tender ones and they love to nick the veggies before you get to them!
How high would my garden fences be? I think they are at lowest maybe just over 5 feet. is this ok?
Clip the wings, both of them, and this should do. Keep nothing close to the fence that a chicken could hop up on and from there make the leap to the fence. Trim all shrubbery from around the perimeter. You could also add 2 ft to the fence.
my other family members are worried about noise and the neighbours? Due to living in Britain, our neighbours our right next door on both sides but we have no one behind us. would some hens be a regular glee club?
Well, in a word, yes. Chickens are social creatures. Will it be bothersome? Most likely not. Hens are normally pretty sedate in a group, although not dead quiet. SO you can expect some "begaaaaawking" from time to time.
And there is often one who will become dominant (alpha) and assume a cocks role. That one can "hencrow" loud enough to wake the dead, but that doesn't always happen - so take things one step at a time.
The Brits love their gardens, and have a bit more heritage with garden-bound city chickens than we
Yanks do. I suggets you speak with the neighbors concerned ahead of time. While it may seem cheeky, it will clear the air and they may even like the idea.
A fun book about chickens in a British gardenscape is called "Hen and the Art of Chicken Maintenance: Reflections on Raising Chickens," by Martin Gurdon.
You should look it up and give it a go.