Great first series...but continuity issues? and why?

So, can someone enlighten me here;
I recently watched all series and parts of Messiah (with the excellent Ken Stott in them and marc Warren in the final installment), I can't remember exactly how I came upon any knowledge of the series but i really loved watching Messiah.
But just reflecting on the first series; I love Messiah, and Ken Stott is now one of my favorite actors and I hope he returns to the role, but were'nt there a few production-continuity issues....for instance; Stott seemed to have more weight on him in certain parts of Messiah than in other parts, and not in a linear-progressive fashion, as in = one scene he would look a certain way and the next scene he'd suddenly have gained a stone??!

Can some one enlighten me as to why this was? was it due to reshooting some scenes later in the year? Did Stott gain or lose 1 or 2 stone as the production went on?
Again, this is just a minor gripe as I loved the series! But it intrigues me as someone wanting to have a future career in TV and film production.