Here's how DAC Bevan is described in Wikipedia:
Deputy Assistant Commissioner Donald Bevan is the team's boss during the first series. He knows both Jack Halford and Gerry Standing, and strongly opposed Standing's inclusion in the team, mainly due to the history between them. (It is later revealed that Gerry punched Bevan in the face and broke his jaw).
I always wonder why the creators of a show are so married to a character's name at this juncture in a show's development. The audience has no attachment to the character yet, so if you're going to recast the role, why not just create a new name, a new character to fill the same purpose?
In the long view it just throws the audience out of the show's believability if they're watching the series on DVD. One episode you've got Tim Woodward in the role of Bevan, and the next it's Nicholas Day. And it's not like you can claim a quantum leap or some other sci-fi splainin for Bevan being a recastaway; the show is too realistic (considering). Woodward and Day aren't even the same height or build, so a more grounded splainin of plastic surgery won't fly.
There's only one way to go with this splainin: they were both Donald Bevan.
It's just one of those quirks in the world, either real or Toob, that there will always be people sharing the same name. I was born a Thomas O'Brien, and that's very common. In my hometown there were about six of them, one of them in my high school class. In fact, I'm Thomas O'Brien III, which is why they were calling me Toby (my initials sounded out) even before I was born - it would be less confusing than calling for Tom O'Brien and having more than one answer. Of course, if my grandfather replied, you better run screaming, as he passed away before my parents even met......
But enough about me.......
In the real world, Donald Bevan is a writer who co-wrote the 1951 play "Stalag 17" and he wrote an episode of 'Producer's Showcase' in 1955. He was also a caricaturist for Sardi's Restaurant in New York City's theatre district. It seems a pretty thin resume to become somebody who inspires the naming of a baby, but as he exists in Toobworld (thanks to several shows in which he was interviewed), it's a pozz'bility. But not one we're committed to......
So let's just say it was one of those flukes in which both were given the name Donald Bevan at birth. And in one of those "Believe It Or Not" twists of Fate, both of them joined the Metropolitan Police in London and eventually rose to the rank of Deputy Assistant Commissioner.
Another thing they both had in common - at some point in their careers, they pissed off Gerry Standing to the extent where the detective punched them in the jaw and broke it.
At some point between March of 2003 and April of 2004, DAC Bevan (as played by Tim Woodward), no longer was in charge of overseeing UCOS. Instead that duty fell to a fellow DAC, the other Donald Bevan.
From that point on, we know Donald Bevan II's history in Toobworld until he was phased out after six episodes. But as for Donald Bevan I, it's time for this televisiologist to go out on a limb.....
It's pozz'ble, just pozz'ble, that DAC Bevan got himself in some sort of trouble on the job which demanded disciplinary action. As a result he lost the position of Deputy Assistant Commissioner and was reduced to Detective Chief Inspector. For alls I know, he was happy with that, as being a DAC meant he was mostly chained to a desk with a sea of paperwork and public relations problems. As a DCI, Bevan could keep his hand in when it came to the reason why he became a copper in the first place - to solve crimes.
At the same time, Donald Bevan I may have wanted to sever all ties to whatever got him into trouble in the first place. And as he was probably fed up with being confused with the other DAC Donald Bevan for so long, he may have petitioned the courts to legally change his name.
And so by March of 2004, one year after creating the UCOS squad, former DAC Donald Bevan became DCI Sebastian Turner, who oversaw the operations of a homicide detective squad, as seen in 'Murder City'..........* This not only splains away the recastaway problem, but it also combines two Tim Woodward characters into one.