We only catch a glimpse of Sheridan, son of Hyacinth and Richard, once in a taxi as it leaves. Normally he is either mentioned in passing or he calls on the phone. He has a reputation for being very dependent on his parents, evidenced by Richard's comment of "No more money this month" whenever their son calls.
The following two years' one-off extended self-contained Christmas Specials, after this "Let there be light" episode, referred to as "Sea Fever" and "Angel Gabriel Blue",can unofficially be considered to be episodes 8 and episode 9 of series 4, as they happen before the start of series 5. As they are the middle Christmas Specials 2 and 3 of the 4 Specials, they are usually skipped over from standard showings.
For Onslow's dog to leap up at Hyacinth through his Ford Cortina Mk4 rear door window at the Church Hall, the glass will have to have been removed as on that model car it didn't wind down completely, in fact only about half way, making it impossible for the large dog to pounce to great comic effect.