Looks like a 11.9hp (12/24) four cylinder. Even if the photo was taken 1928, the car is certainly earlier. It's from the early 20's, probably 1921, with the upright windscreen, fitted bulb horn, side lamp etc of that time. The opening two-pane screen had now been introduced. The registration number backs this up - the XH prefix was issued in London and, I believe, only for a short time (March 1921 to January 1922). The car is not in the AC vintage register, so must have passed to that great parking lot in the sky....