Actually as far as I recall, the advertising I read at the time during around a year before and until everything was closed to organise the gig, it said it was a gig where Silver Cliff Award Winners will perform, and DS were one, and that's why they were advertised to play Knebworth.
Then, when closing the acts, some bands that won the award didn't play, some others that didn't won like Tears for fears played, Robert Plant that just won it the same year played it, and that's what you have on official merchandise. What they closed in the end.
And the fact is also, when rehearsing, it was DS who rehearsed for the gig.
The fact official merchandise didn't reflect that when the band rehearsing for the gig was DS (MK, John and Guy with Clapton band of course) is for me more a mistake of someone because of the fact it was a supergroup with Clapton, Knopfler and Elton John.
But ok, youre right