Lorraine has dropped me a line:
I'm interested in your "introduction", in which you state that Seth Armstrong became a full-time regular in the 1980s. Surely Seth became a full-time regular in the 1970s?
No, I'm afraid he didn't. Stan Richards debuted as Seth in 1978, the character was then a school caretaker. It was supposed to be a one-off story-line, but the Emmerdale Farm production team liked the character, so Seth appeared in several story-lines afterwards - a particularly memorable one being his employment as gamekeeper for NY Estates.
But for large tracts of episodes in 1978, 1979 and early 1980 Seth simply didn't appear at all and was not referred to. Producer Anne W Gibbons made the decision that the character should become full-time, which he did in mid-1980. From then on, Seth was a Woolpack regular - and on-screen as much as any of the other major characters.
My knowledge is based on watching the episodes. When I wrote that Seth Armstrong became a full-time regular character in the 1980s, that's exactly what I meant. He did - in mid-1980.