Continuity mistake: In the scene where Brian is in the courtyard talking to Keny senior, he quickly changes from his shopmobility to his wheelchair. This happens between the shot of Keny on the roof. [It is stated in the commentary that it is part of a storyline that was abandoned (hence still a mistake, but with a reason). The idea was that every time you turned your back on Brian he could actually walk, hence him getting into his chair so quickly. This storyline was abandoned though later bought to light when Brian was investigated by the DSS.]
Other mistake: At the start of the line dancing scene where Ray Von is deejaying, if you have the subtitles on, it says the song is Hillbilly Rock, Hillbilly Roll by the Woolpackers. It's actually 5,6,7,8 by Steps. (I'm quite embarrassed I know that).
Deliberate mistake: In the scene with Brian and Beverly going up the stairs, Brian is in the chairlift, and you can see in front of the wig that Peter Kay wears where the red patch is where he has shaved his real hair.
Answer: Yes it is PK - the characters he plays are Brian, Max, Keith Lard and indeed the voice of the radio. you can hear him on the radio a few times throughout the series.