The filming of Five On A Hike Together for this series was done at Woodchester Mansion and Park near Stroud, Gloucs - an unfinished 1860s Gothic country house that has a reputation for being haunted, appropriately for a FF mystery. This is some miles NE of the HTV studios in Bristol where the second series was based, and the wooded deep Cotswold valleys can be seen in the shots. It's a few miles from the village and valley of Slad where the writer Laurie Lee grew up, which I have been to; 'Cider With Rosie' is one of my favourite books and I was thrilled to see LL's home and school. Very picturesque, but a nightmare to get to when snow or ice is on the roads!
The view of the house as the children arrive there at 12 mins, 25 secs into the YouTube download of Hike Together can be compared to a photo of the mansion taken from the same angle on the '
www.geograph.org.uk' website, at square S08001/ photo 5179364 by Philip Halling - the gables, roofs, windows etc are identical so this is clearly it.
The interior of the building is at square S08001/ photo 116861 by David Exworth, cf shot of the children in the same passageway at 12 mins 34 secs in to the film.
'Gloomy Water', aka the Middle Pond at Woodchester Mansion's park, is at square SO8201/ photo 931925 by Brian Robert Marshall and ibid , photo 6311107 by Philip Hallong; cf film at 17 mins 34 secs into the episode of the children at the lake.
The 1978 version at Sowley Pond is perhaps more exciting, with the villains' boat capsizing in the middle of the lake: did 'Health and Safety' fears stop this in 1995? In the more recent version, we also just have George wading across the lake to find 'Saucy Jane' in the bushes not any diving!