DETECTIVE Chief Inspector Frank Burnside is back, but with a difference. He's quit the Sun Hill cop shop he inhabited in ITV's The Bill and is pounding a new beat. He has his own series called - what else? - Burnside.

"Television's toughest and most uncompromising detective" is now leading a crack police team in a crime-busting outfit called the National Crime Squad. He's also heading an attempt by schedulers to squeeze more mileage out of the franchise known as The Bill.

Since its revamp a couple of years ago, the long-running police series has reversed flagging ratings and re-established itself as a reliable audience grabber. Burnside is the latest attempt to cash in on that popularity with the spin-off providing ideal material to fill the wide open spaces of the ITV evening schedules left vacant since the demise of News At Ten. The idea must have been tempting too for Christopher Ellison, who plays Burnside, as Ellington, the series in which he starred after leaving The Bill, was not a success.

For ITV bosses, the spin-off has instant audience appeal. No need to educate viewers about a new character. This is one occasion when familiarity breeds not contempt but instant ratings. A bonus is that in a post-watershed slot Burnside can be even tougher, meaner and foul-mouthed than he was in The Bill's 8pm slot.

He's not the first detective to win his own series. Back in the 1960s Charlie Barlow and John Watts (Stratford Johns and Frank Windsor) left Z Cars and Newtown for the Regional Crime Squad and their own series Softly, Softly.

Crime isn't the only subject that pays dividends in the spin-off market. After a decade or so on BBC screens, hospital drama Casualty gave birth to Holby City which followed patients from the accident and emergency unit in the former series into the hospital wards in the latter. Characters from one series turn up from time to time in the spin-off to reinforce the connection. The XYY Man was a forgettable ITV series which ran for two years in the 1970s but did result in one of the main characters Detective Sergeant George Bulman appearing in not one but two spin-offs. The glove-wearing, Shakespeare-quoting detective went on to Strangers (moving from London's Scotland Yard to a Manchester crime beat) and then on to his own series Bulman. By now Bulman, played by Don Henderson, had left the police force to run an antiques shop in London but was persuaded to put his sleuthing skills to good use as a private eye.

The popular period drama Upstairs Downstairs led to two of its most popular characters Thomas And Sarah lending their names to a spin-off following the chauffeur and the parlour maid, played by real life couple John Alderton and Pauline Collins, as they left London to take up new jobs in the country.

Soaps like Coronation Street, EastEnders, Emmerdale, Brookside and Hollyoaks have tended to extend their territory with specially-made videos rather than spin-off series. An exception was Damon And Debbie which followed the young lovers from the Liverpool Close as they went on the run from police in York. In another series South, shown in Channel 4's The English Programme, Tracy Corkhill and her boyfriend Jamie experienced the bright lights of London.

The glossy US soap Dallas led to Knots Landing about the life of Ewing black sheep Gary in a Californian cul-de-sac. Dynasty gave us the equally glamorous The Colbys (although few viewers actually wanted it).

Just occasionally a character from a sit-com turns up again being serious rather than funny. That happened to Ed Asner's grumpy newspaper man who moved from sit-com The Mary Tyler Moore Show to his own drama series Lou Grant.

Comedy has proved even more profitable than drama for spin-offs - apart from the little-remembered comedy Don't Drink The Water in which Stephen Lewis's Inspector Blakey character from On The Buses retired to Spain with his spinster sister, played by Pat Coombs.

One of the earliest spin-offs 40 years ago saw The Army Game followed by Bootsie And Snudge in which the two much-loved characters (played by Bill Fraser and Alfie Bass) tried to cope with life back in civvie street.

Man About The House, the comedy about the flat-sharing trio, led to two spin-off shows.

First George And Mildred which centred around the life of their landlords the Ropers (Brian Murphy and Yootha Joyce) on a suburban housing estate while Robin's Nest followed when Richard O'Sullivan's Robin Tripp opened his own bistro. Classroom comedy Please Sir left school to follow the pupils' fortunes in the outside world in The Fenn Street Gang. One of the most popular parts of Canned Carrott resulted in The Detectives, alias Jasper Carrott and Robert Powell, getting a series of their own.

The Americans haven't been slow in presenting comic spin-offs. One of its most successful shows, Frasier, had its origins in the bar room comedy Cheers. The new series simply followed Kelsey Grammar's radio psychiatrist as he moved from Boston to Seattle.

You can catch Burnside on ITV, Thursday at 9pm.