Visible crew/equipment: When the Hamiltons arrive, Mrs Hamilton says 'May I introduce my husband.' The boom mic is seen at the top of the screen.
Continuity mistake: When Basil is talking to Mr. Hamilton in the dining room there is smoke coming from the kitchen. The next moment he goes into the kitchen and all the smoke is gone. (00:24:00 - 00:25:00)
Visible crew/equipment: When the Hamilton's are talking to Sybil, Basil enters and Mrs Hamilton says 'Harry finds it too gloomy'. At the top of the screen the boom mic is visible.
Visible crew/equipment: When Basil says to Mr. Hamilton "I'm sorry if it wasn't wide enough for you, a lot of the English cars have steering wheels" the boom mic can be seen in the top left corner.
Continuity mistake: When Basil brings the Hamilton's their bottle of wine, he fills up Mr Hamilton's glass but not Mrs Hamilton's. However, a couple of shots later, we see both Mr and Mrs Hamilton drinking wine from their glasses despite nobody pouring any wine into Mrs Hamilton's glass.
Answer: It's "Leisure Complex" by Dave Gold and Gordon Rees. Can be found on YouTube. The relevant clip is around 25 seconds in.