There's nothing more annoying for the viewer than to have a series they've been following - at a time they are able to watch - suddenly shifted around the schedules half way through. So why do the programmers do it?

THE producer of a new BBC2 documentary series called Channel 4 schedulers a rude word when I informed her they planned to screen the old Tony Curtis and Kirk Douglas movie The Vikings last weekend. It appears the channel had been trying to find out for ages when Blood Of The Vikings, one of the highlights of BBC2 autumn season, was due to start so they could show the film and, no doubt, add to the viewing figures, but basking in the hype for the BBC programmes.

The Vikings' reputation for rape and pillage is nothing compared to the cut-throat business of TV scheduling these days.

Blood Of The Vikings began on Tuesday, two weeks earlier than originally planned - a victim of the chopping and changing at the last minute that schedulers, notably on the BBC and ITV, indulge in these days.

Alan Yentob, former BBC1 controller, was famous for dithering and not deciding until the very last minute -when the TV listings magazines are put to bed - what he was going to show when. His successor Lorraine Heggessey is following in his faltering footsteps.

She sends me (and hundreds of other journalists, I don't want you to think I get preferential treatment) an advance tape of the first three episodes of the Liza Tarbuck show Linda Green, one of BBC1's great hopes for the autumn. What, of course, the accompanying letter didn't state is when the series was beginning. That would be telling and give ITV a chance to schedule a strong programme, or spoiler, against it.

At a BBC seasonal launch earlier this year the cast of police series Mersey Beat was there to be interviewed. Ms Heggessey came across to wish the actors luck while I was interviewing them. "When is the series starting?," was my not-unreasonable question to her. The answer was not enlightening. "I know which week but not which day -- and I'm not telling you anyway," she replied. Clearly she has attended classes at the James Bond school of sealed-lipped skullduggery.

Over at ITV, they are no less secretive. Programmes are launched in advance in the hope that they'll eventually be shown. The stars attend and are interviewed by the press. The first episode or highlights are shown to give an idea of what's coming. Then it all changes. Between the London launch and the first screening, the Albert Finney series My Uncle Silas switched from six half-hour programmes to three one-hours.

Even ITV's top stars suffer. The starting date of Robson Green's Take Me was cancelled at least once after the press preview. It was even listed in the Granada press information pack before being postponed until a later date.

With advertising revenue down, ITV needs to maximise its ratings. If a series doesn't pull in sufficient viewers, it's unceremoniously dumped or moved to a later time slot, much to the annoyance of those viewers who are hooked. These people can play a game of Hunt Your Favourite TV Show, searching the schedules trying to find where the last episode of the show they've watched religiously for five weeks has been relegated.

Usually an old Frost police drama with David Jason or a James Bond movie is slotted into the gap vacated by the dumped programme. Both are surefire ratings winners no matter how many times they've been shown.

Ardal O'Hanlon's Big Bad World series simply disappeared after several episodes because ratings were disappointing. Some months later the series was finished off in a late, late slot. Bob And Rose, the new six-parter from Queer As Folk writer Russell T Davies, was brought forward from the planned November slot because ITV chiefs were so certain they had a winner on their hands. When ratings failed to rise above the four million mark, not good for a peak time drama series, they lost their enthusiasm. The final part was shifted post-News At Ten to 10.20pm. Its place at 9pm was replaced by a tired but ratings-friendly Denis Norden Laughter File. Ross Kemp's Without Motive suffered a similar fate as the final two parts were moved to that time spot.

Tomorrow Cilla Black marches victoriously back into the 7pm slot on ITV with another - I refuse to call the 17th series of anything, new - series of Blind Date. Poor Cilla is under the misapprehension that she was responsible for booting Des Lynam and his footie-crazy boys show The Premiership out of peaktime and into a post-10pm position. She may claim "a victory for the nation's women" (as if some females don't like football) but ditching Des was a commercial, not an emotional decision. Advertisers were unhappy with the poor ratings, around four million, for the show.

Network director David Liddiment admitted, when The Premiership was launched in September, that ratings would take time to build. On reflection, perhaps it wasn't wise to boast that this would change the face of Saturday night TV forever, but it was a bold move to fill a large chunk of the peaktime ITV weekend with sport. Maybe we should be praising Liddiment for his courage rather than taking him to task for failing to attract enough viewers. It's not long ago that people were complaining that Saturday night was a desert for good shows.

The schedule of light entertainment hasn't changed much for 25 years. Now that Cilla's back, it's unlikely to change for another 25 years. We are, I fear, condemned to a lorra lorra shows, like Blind Date, Casualty and The Generation Game, going on and on and on and on...