I've just checked the information on your site about The Cure's concert on October 9th 1979 at the Gaumont Theatre in Ipswich supporting Siouxsie and the Banshees. You have a note that, (...) , this show was cancelled - I attended the concert and am sure they played, as well as Robert Smith playing with Siouxsie due to the departure of John McKay.
The concert is in the list I wrote of gigs which I attended, with no mention of the band not performing, so even though my memory is a little vague, I'm sure The Cure did perform before Siouxsie. I still have the Join Hands tour programme which I bought at the concert, though it got rather damaged due to being held in sweaty hands for a few hours!
from Jonathan V.:
The gig at the Ipswich Gaumont in 1979 wasn't cancelled and Robert Smith did play guitar with the Banshees that night.