the actor who plays Patrick Troughton

Yes, what a shame. I like Reece Sheersmith, but he was nothing like Troughton, it's the only bad bit of casting in the programme and was all the more noticeable because of the superb casting of everyone else.

I was expecting to see David Troughton playing the part of his dad, but maybe he's a bit too old now to play it, or maybe it was just too wierd a concept, but in terms of looks he's a dead-ringer.

Nope I also kind of cringed when he came on scene. But I think the actor was hired for his ability to impersonate the voice and mannerisms which he did wonderfully. I guess they were left with a choice of casting someone who looks like Troughton or someone who acts like Troughton and they went with the latter.

Lets be honest....Shearsmith is an old buddy of Mark Gatiss who wrote the script so maybe not such an unusual bit of casting?

Agreed. I was very disappointed that he looked nothing like Troughton, especially since so much of the casting was spot on. However, he did nail the voice and body language.

I'd like to be a pessimist, but this is a luxury I cannot afford.
—Joseph of Cordoba

Eh, sure it's nice for an actor playing a real person to resemble them, but it's not remotely as important as the ability to act as them. Check The King's Speech with Colin Firth's turn as King George VI, who look absolutely nothing like each other, save perhaps skin colour.

He looked way too young to me, I have seen some of the old Patrick Troughton episodes and he appeared to be a bit older than the guy they showed here.

Just watched this and yes, I agree this was awful miscasting and it threw me out of that scene. Gatiss should have been more professional and cast someone more suitable.

Looks nothing like Trougton, but I agree that with the wig, clothes, and a very good attempt at an impersonation, it works. It would be very tough to find an actor that could pull off all three points to be a dead ringer for Troughton. The real question was the bonus scene on the DVD that cobbled together a hokey regeneration scene from Troughton to Pertwee. We see Toughton lying unconscious, then a fade in to white, and then a fade out to Pertwee, who then starts to blink his eyes open, and then open them wide in amazement. They botched this because Pertwee was wearing a different tie and collar. The Doctor regenerates, not his clothes. Was this regeneration scene played by the same actor, or two different actors? It's not listed on the credits.