About three miles north-east of the famous Pinewood Studios lies Harefield Grove Farm, a small estate originally comprising a manor house, outbuildings, barns and large greenhouses (in which exotic fruits were cultivated). Given the local area's thriving television industry, the estate's owners saw an opportunity to raise much-needed revenue as they bailed out of British farming's downward spiral during the 1970s. Indeed series such as The Adventures of Black Beauty, Dave Allen At Large and The Two Ronnies had rented the premises for location filming at varios times.

Mark 1 Productions (or Avengers Film & TV Enterprises as they were known at the time) had been using Pinewood as a base for The New Avengers but the agreement hinged on the studio having a share in that show's profits. Although London Weekend Television was backing The Professionals, their own studio resources were not part of the deal and so Mark 1 had to make their own arrangements. When Harefield Grove became available for rental, Mark 1 jumped at the offer, reportedly paying the landowners the princely sum of GBP100 a day!

Harefield Grove's buildings not only provided many of the locations and interiors seen in the first season but a full editing suite and other offices were also constructed. Although LWT was injecting GBP1.5m into the first season, this wasn't really sufficient for 13 episodes. Each season of The New Avengers had cost over GBP2m and its own financial difficulties meant Mark 1 had yet to make any profit on that series. Budgeting was, therefore, "tighter than a spinster's girdle" (as Bodie might say). As such locations throughout the Grove are re-used numerous times which, of course, also had the benefit of keeping transportation costs down.

With the huge success of the first season, the superior editing and office facilities at Lee International Studios in Wembley were offered to the production and the crew never returned to the Grove. In 1985 the 18th-century manor house underwent restoration but was converted into offices. The entire estate was eventually sold off.

As with many locations used in the series, much has changed at the Grove in the intervening years, particularly with the unconvertible farming-specific buildings being understandably removed. The main house, however, is a listed building and therefore cannot be demolished.

There is no public access to the site now and the photos below were taken after negotiating special permission.

Exterior shots reveal locations used for the tin-shooting scene in 'Private Madness, Public Danger' and the Star Hotel in 'Everest Was Also Conquered'.

The grounds originally had a small mews and stables. These were used as Joanna's house in 'Female Factor' and Doyle's place in 'When the Heat Cools Off' respectively. All demolished since, however.

Scenes inside CI5 headquarters used the main mansion house. The various briefing rooms, corridors and interrogation areas were all shot here. As noted, the interior has since been partitioned into separate offices, so little is now recognisable. The main stairwell that Bodie and Doyle race up and down in several episodes remains (though substantially refitted) but the lift has been removed.

This site had its own indoor squash court... which craftily doubled as a prison visitors area in 'Old Dog With New Tricks' and the gym in 'Long Shot'.

The wine cellars became Cusack's little armoury in 'Where the Jungle Ends'.