Although very little is known about this show, one source (thebritishsound.blogspot.co.uk) confirms it did go ahead. This would have been Tony Kaye's first live performance with the group, it is unclear whether Clive Bayley had left by this show or whether it was his last show. (Anon.)
From the Mabel Greer's Toyshop Facebook page on 3rd August 2018:
"Following Bob Hagger’s departure in June, Bayley, Squire, Banks, Anderson, Kaye and Bruford performed the last documented shows as Mabel Greer’s Toyshop on June 7, 1968 in London (Bruford had joined the band the same day) and in Kingston-Upon-Thames, Surrey on July 20 (the after-party enjoyed at Clive’s parents house in Kingston). From August 1968, after 2 years as Mabel Greer’s Toyshop, the band adopted the name of ‘Yes’. A label less imaginative, but easier to remember. It was at the Lucky Horseshoe that Clive decided it was time to move on. His baby had grown up and flown the nest…"