Burly? Don't think we saw that much of the intruder, and Gareth has probably grown since we last saw him (when the fire happened). When he was in the office with Glenda, Terry and the social worker, he seemed a lot older, and they grow quickly at that age. I'm not sure if we saw him standing up.
It seems highly likely to me! He's fairly resourceful, when he wants to be. He could possibly have scrounged a lift with a lorry driver or something, or stolen the money for the train fare.
But what does he want in Glanrafon, I wonder?