It's a 27mm Mauser -
https://en.m.wikipedia.org/wiki/Mauser_BK-27 - not an Oerlikon!
Apart from that very good recollection from RPE. He's always been a top chap. Not so sure about the interviewer's questioning technique, though - lot's of 'yes/no' questions. Also they had left a paper coffee cup in shot for the first bit.
It is great to see stuff like this being recorded for years to come. I heard today of a project that RAFA are doing to digitally record veteran RAF members in order to feed the National Archives.
LJ