how much more growth does a 2 year old pony have?

Native ponies tend to be almost fully grown at 2. He will probably grow 2 or 3 inches more. But then his body will start filling out and getting more muscular (particularly around the shoulders if dartmoors are anything like Welsh ponies or tend to be pear shaped until they are about 2 or 3).

as a dartmoor the pony from 11.2 should make 12.1 maybe 2, cant see making 13hh or higher

Generally not much but it does depend on the horse (bloodlines). I have one mare whose foals are teeny and ugly until they hit about 2.5 then woosh they grow like weeds and fill out everywhere and turn from ducklings into swans. Ask the folks who own the pony's mom what her foals tend to do growthwise.

well, once the horse or pony is 2 years of age, it doesn't grow much after that. You would be lucky if the pony was 12 or 12.1 hh when done growing!