Guarian of the Abyss
Old Amersham High Street. You can clearly see The Kings Arms Hotel (As used in Four Weddings & A Funeral).
Simon Andrew's antique shop next to the Kings Arms hotel
The Thirteenth Reunion
The Kings Arms Hotel was used for the restaurant scene when Ben and Ruth have a meal. Thanks to Kevin Flanagan for confirming this location.
Visitor From The Grave
The lake where the car is dumped is almost certainly the lake at Shardloes. Thanks to Kevin Flanagan for confirming.