It's unclear how many episodes he'll be in. Yes, he came back to do 17P, but he also appears to have gone back to the UK a couple of times. He was contracted for three years on the show, though, so time will tell.
Don't forget, ITV airs the episode orders split, so we're likely to only get seven of the 13 starting in July. If BBC America picks up the show on or about the same time, they may air them all... no way of telling until somebody stops being cagey and fesses up. ;)