I like Alexander Armstrong and I think he'll do justice to the voice of Danger Mouse. His style is very similar to that of David Jason. Dave Lamb (in voiceover mode) makes anything sound exciting. A good choice. It is stated in the original BBC Press Release that "Further cast announcements for additional characters will be made at a later date". Edward Kelsey did the voices of both Baron Greenback and Colonel K in the original cartoon, now apparently a regular on The Archers.
Not too keen on the new look of Danger Mouse from the BBC article above, as to look at it infers it's going to look like The Beano did when it relaunched itself a few years ago and started looking like Pokemon on speed.
Time will tell whether the new Danger Mouse is any good. I hope it is.
There are lots of reasons why David Jason might not have been offered the role. At 74 they might have thought it was not a very good long term bet. As mentioned he could have been too expensive or not interested in project. They might have thought his voice didn't fit the new vision of the character.
It's not as if they are producing this for fans of the original.
Plus at his voice now isn't the same as it was 30 years ago. Alexander Armstrong seems a good choice.
Obviously this is being made with CBBC in mind but they'd be wise to use is as a bit of filler in the early Saturday or Sunday schedule when times allow - be interesting to see how it did.