An antique mirror turns up at a house clearance. A young girl on the run from a cult society claims the the mirror is a scrying glass used to summon Chorozon, the almighty devil. Soon the society led by Charles Randolph are keen to get their hands on both the mirror and the girl. [full story]
Production information for Guardian of The Abyss
15th November 1980 (UK ITV)
Laura Stephen's antique shop was located in Hill Avenue, Amersham-on-the-Hill, about 5 shops down from the old Post Office (I think this may be the same shop as used in Charlie Boy for Heinz's antique shop).
Simon Andrews antique shop was in Old Amersham High Street, next to the Kings Arms hotel.
When Mike stops to pick up Allison, you can see Hampden House in the distance with what must have been at one time, a gatehouse. This is on Rignall Road leading from Gt. Missenden to Chequers.
Hampden House is used extensively throughout this episode as the Chorozson Society headquarters. You get to see some nice shots of the exterior and interior.
I'm sure that the shots used for the interior of Mike's house are infact Hampden House as when Randolph comes to see Mike, you can see the shaped conifer trees through the window that you also see when Allison is escaping.
*update* However Andy Pagin, a visitor to the site and Hampden House is certain Hampden House wasn't used for the interior shots of Mike's house