“Peter Zinovieff had his first official contact with the BBC Radiophonic Workshop on October 28, 1964, when he was given a tour of the Workshop's facilities in his capacity as a Canadian Broadcasting Corporation employee.”
On 15th January 1968, she can be seen helping him set off his Partita for Unattended Computer as part of a concert of electronic music including her own piece Pot Pourri at the Royal Festival Hall.
Shortly before Delia died, she wrote the following (of Sonic Boom):
One of our first points of contact - the visionary work of Peter Zinovieff - has touched us both, and has been an inspiration.
though in 2010, Peter Zinovieff referred to her as his 'assistant'
Interviewer: Do we hear that right? Delia Derbyshire was your assistant at some stage?
Zinovieff: Yes. She -er - I formed a
Interviewer: Lucky you!
Zinovieff: Oh! I formed a group called Unit Delta Plus and I got Delia Derbyshire and Brian Hodgson from the BBC to join me. Um. The idea was that we would make a fortune doing commercial sounds and I wasn't interested in doing commercial sounds. We did one commercial sound called Philips which was something like “wup!” and that was it. We got a lot of money for that but I didn't want to do that so we split after a bit and they went on. But they didn't succeed either. 
I tried to communicate with Delia and I also tried to communicate with Mendelson[?] but Mendelson was no good; he just said "Get on with it" and when I suggested to Delia that I'd like to try and do something as if we had collaborated 50 or whatever it was years ago, she said "Well, just get on with it however you like."
and I am told that in private, he says:
She was only interested in money.