This is actually a pretty recent bootleg, but the content is vintage Clash.

The cover tells the whole story: What we have here is the Only Band That Matters’ May 28, 1977 show at De Montford Hall in Leicester. If I had a time machine, I’d probably burn it on this — not killing Hitler, not saving Will and Holly from Chaka, but on a chance to see the Clash on their 1977 tour. This isn’t footage from the De Montford Hall gig, but I present it to you as exhibit A that this would be the appropriate use of a time machine:

There are a few versions of this show floating around on bootlegs, but this one will set you back 25-50 bucks. Happy hunting.