Another Episode Question

I live in the USA and have not watched Rising Damp yet. So I am wondering...
In which series and episode is Don Warrington's Character first introduced?

Those perplexed by my Rising Damp gallery and the above statement/question
may be interested to know that I captured the stills for that gallery from a Leonard
Rossiter documentary that was featured on my Fall and Rise of Reginald Perrin
region-1 DVD set.

Don appears in all the episodes.
He is first introduced in 'Rooksby' the first episode as Philip Smith, a student miss jones recommends for the room downdstaris