CASUALTY may be one of the BBC's most watched, longest-running drama serials but nothing can be taken for granted in the modern TV world. So when Darlington-born Mervyn Watson was appointed series producer he sat down and took a long hard look at the Holby Accident and Emergency operation.

"Television is such a competitive industry now, it doesn't matter what it is and how long it's been running, a series has to fight for its slot and right to exist," says the former producer of Coronation Street and Emmerdale.

The peak-time slot occupied by Casualty for much of the year is a key one in the fight for Saturday night ratings supremacy, with the arrival of ITV's football programme The Premiership signalling a shake-up of the schedules.

Watson returned to the BBC after working as MD of small independent production outfit Red Rooster. When that was absorbed into a bigger company, he found himself seeking a return to hands-on producing.

"I found myself looking for a show," he says. "For the previous two or three years I had been doing executive stuff at some distance from the actual shows.

"When Mal Young (BBC Controller of Drama Series) said, 'how about Casualty?', I leapt at it because it's high profile and one of the BBC's most important series."

As the BBC's deputy head of drama series a decade ago, the hospital series had been in his charge although he took more of an overview rather than being concerned with the day-to-day running of Casualty.

One challenge was that the 16th series, which begins tomorrow, will run longer than ever before but such is the competitive nature of TV that Watson is unwilling to say exactly how many episodes have been made.

"There were 33 last year and more this year," is as precise as he'll get. With the number of episodes for Casualty and its spin-off Holby City increased, both are creeping ever nearer to year-round soap status. My approach was to say that this is not series 16 of a very established series and take it for granted, but to say this is series one."

Casualty itself is filmed in Bristol with the story machine, as he calls it, in London. He's series producer, with two individual producers working for him, overseeing the episodes.

While away from the BBC, he has "dropped in" on Casualty without being a regular follower. "I was aware of it and the regular characters that had been in for a long time. I know some of the people who had come to it, like Ian Kelsey whom I worked with on Emmerdale," he says.

"Basically, doing a large number of episodes means you can't afford to get into the trap of having repetitive medical stories that eventually begin to look the same.

"I feel what people come back to in a series like Casualty are the regular characters, so I have upped their stories. We have the same size of cast but some have changed. We've said goodbye to five or six people and said hello to an equal number."

His aims included giving regular characters dynamic storylines, increasing the pace to bring it up to speed with other TV drama and injecting humour into the format.

"I don't mean funny stories because it's serious things that happen in hospital. But every audience loves character humour and there's a lot of black humour around medical staff," he explains.

"I wanted to increase the serial element, to make sure the stories of incoming characters affected the regulars as well. We wanted variety, to avoid a pat regular formula."

Since Casualty began, it's been joined on TV screens by other hospital series such as Holby City and A and E but Watson still finds it an exciting programme to work on.

There will be occasional crossovers between Casualty and Holby. Practically that's difficult to organise as the two production centres, Casualty in Bristol and Holby in Elstree, are 130 miles apart. "We regularly exchange proposed storylines so we don't end up looking the same in the same week," says Watson.

Poor early ratings for The Premiership on ITV would seem to leave the way clear for Casualty to continue to dominate the Saturday ratings. Watson won't be drawn on this as the subject comes within the domain of BBC strategy. "I'm just concerned with making the programme as good as I can whatever time slot it's in," he adds.

* Casualty returns to BBC1 tomorrow at 8.10pm.