Mervyn Bunter is the gentleman's gentleman serving Lord Peter Wimsey in the stories by Dorothy L. Sayers. In the Television Universe, he was first played by Ronald Adam in a 1947 adaptation of "Busman's Holiday" - which was then adapted again a decade later. In that version, Charles Lloyd Pack portrayed Bunter.
Adam would have been considered the official version of Mervyn Bunter, being the first, following the general rule of Toobworld Central. However, both Adam and Lloyd Pack only appeared in those single adaptations, and Toobworld Central always leans toward a portrayal of some heft - in this type of case, quantity does over-ride quality. And therefore Glyn Houston has the honor of being the Mervyn Bunter of Earth Prime-Time. That his lordship Peter Wimsey as played by Ian Carmichael is also the official version in the main Toobworld helps his case.
However, Bunter, as seen during Lord Wimsey's investigation into "The Unpleasantness Of The Bellona Club", looked markedly different than he did while serving his lordship in three other murder investigations ("Clouds Of Witness", "The Nine Tailors", and "The Five Red Herrings"). In the real world, this is because Derek Newark played Bunter in that second installment, whereas Glyn Houston was the valet in the other three mysteries. (Bunter does not appear at all in "Murder Must Advertise".)
No need for quantum leapers or alien impersonators in this case. Here's the Toobworld splainin:
For whatever reason, Mervyn Bunter had to leave his master's employ temporarily, in order to take care of personal business. We can look to the sitcom 'Family Affair' for the precedent of the splainin: in much the same way as Nigel French (sometimes referred to as "Niles") covered for his brother Giles, Bunter had his younger brother serve in his stead during that time. (The first Mr. French had to abandon his post with Bill Davis in order to serve at Her Majesty's request.)
Unlike the case of the French brothers in 'Family Affair', the matter of the replacement of one Bunter brother for the other was never addressed in the episode. Not everything has to be shown on TV. Take the example of Singh Noonian Khan meeting Pavel Chekov in the "Space Seed" episode of 'Star Trek'. It never happened, and yet Khan recognized Chekov when they met in the movie "Star Trek: The Wrath Of Khan".This second Bunter is never addressed or referred to by his first name. So we're spared any need to splain why two brothers would have the same first name of Mervyn. But if he had been? Not a problem. Toobworld Central would have invoked the "George Foreman Ploy" (not a title for a "Big Bang Theory' episode!) in which all the siblings have the same first name. This was also the case with Arthur Dales of 'The X-Files'. Both Darrin McGavin and M. Emmett Walsh played characters with that name, and they were brothers. (Apparently they had a sister and once even had a dog by the name of Arthur Dales.)Whatever the younger Bunter's first name, I don't think it was "William". For those who might be familiar with the boys' adventure stories of the early 1900's, those "ripping yarns", the name of Lord Peter Wimsey's valet - Bunter - should bring to mind Billy Bunter, the overweight schoolboy always getting into a jam at the Greyfriar's school in the stories by Frank Richards aka Charles Hamilton.
Billy Bunter, Mervyn Bunter, and his younger, un-named, brother are likely cousins, a familial connection no closer than that.
(Both Billy Bunter and Frank Richards exist in the TV Universe. Richards was abducted to an alien dimension in order to provide the imagination for the Master Brain.)
[And this Wimsey post is dedicated to my brother Bill.....]