I was thinking, Are You Being Served? However, I can only come up with 3, Frank, Trevor and Nicholas.
Add in John Challis who played Capt Peacock in the one off remake of Are You Being Served [Jeremy in our beloved show] and Roy Barraclough who played Mr Grainger in that same remake [ Crowcroft in our beloved show] and I think you are there Electrical so hopefully well done sir!
This is probably not the answer you are looking for but I've been watching All Creatures Great And Small and noticed a lot of LOSW cast members appearing in episodes mainly in minor roles or one off appearances. I've seen Thora Hird, Teddy Turner, Janet Davies, Tony Capstick, Danny O'Dea, Joe Gladwin, John Comer, Ken Kitson, Jean Fergusson and Gordon Wharmby.
So Open all House perhaps Lynda Baron, Maggie Ollerenshaw, Kathy Staff, Janet Davies , John Challis [Jeremy again boy has he been in some shows]. Teddy Turner [Edgar in small Tune on a Penny Wasail] .
Madge Hindle was also in First of the summer Wine
Going back a bit but could it be Six Dates with Barker. It starred Ronnie Barker and David Jason but also Michael Bates, Josephine Tewson, Derek Ware, Alan Curtis and Larry Noble. In LOSW Derek Ware was the painter on the scaffolding tower in Uncle of the Bride. Alan Curtis and Larry Noble played Danny and Mouse in A Quiet Drink.
The oldest show I remember Kathy Staff in was CROSSROADS . However I don't remember any one from SUMMER WINE appearing in it !![]()