The catchy theme tune, "Eye Level", reached number one on the UK charts at the end of 1972 when this show was first shown, and stayed on the chart for twenty-four weeks. It has the distinction of being the first television theme song to top the official UK music chart.
Although exterior location filming took place in Amsterdam, all interior scenes were recorded at Thames Television's studios at Teddington Lock, Middlesex.
When this series first debuted on the ITV network in 1972 it touched on surprisingly adult themes such as pornography, homosexuality, hard drugs and child murder, quite groundbreaking stuff for a UK made TV show at the time. This inevitably meant it had to be shown in the post watershed (9pm) time slot.
Theme tune "Eye Level" proved a very versatile track, being used in advertisements for Oranjeboom lager, KLM Royal Dutch Airlines and Staffordshire theme park Alton Towers. With lyrics by Melvyn Taggart it became the song "And You Smiled", performed by Matt Monro. His single version was released by EMI Records in November 1973 (catalogue number EMI 2091) and was his final big hit, reaching 28 in the UK charts.
Barry Foster, Geoffrey Bayldon, and David McKail are amongst the very few actors to have acted in the 1970s seasons and the 1990s seasons.