Dose anyone know where I could use an underwater Treadmill for my dog.

Never heard of it ??!!   Hope its not a treadmill thats plugged in lol :-)

they have one at my vet in lancashire

Where are you? If you are in the South, Sussex Canine Centre have one they are in Lewes but i am not sure if you need a vets referral for it i know you do for their Hydrotherapy Pools.debby.

there is one at are Hydrotherapy centre where i take my dog we are in lincolnshire.

There's one in Leeds if you are around that area - did my boy the world of good after his 1st cruciate op!

I am in Essex between Chelmsford and Southend.

there is one in east devon too

there's also one in Nuneaton, Warwickshire.

We used one and that was near Bury st Edmunds...

The Hydrotherapy centre I go to has one, it's near Maidstone in Kent. So the wrong side of the river from you, but not a million miles away. This is the link to the Canine Hydrotherapy Association, you might be able to find one closer to you. I had a quick look and there's a member in Chingford, but I didn't look to see what services they offer.

I've heard great things about this piece of equipment, apparently they are great at assessing how your dog is moving.
Normally they are in the centres of Canine Hydrotherapy.
The only one i know of is more than likely the same one Tadsey is talking about in Maidstone, again not in my area but i've heard it being spoken about in a good way.

I would personally after using different pools myself with my dogs for fun use one affiliated with the CHA, link posted above my another poster.