This sort of thing gets harder as the collection gets bigger

Have you ever had one of those annoying moments (or hours...) when you have a snatch of tune going round iin your head but you just can't remember what it comes from? I spent about an hour this evening tapping my foot to the imaginary strains of this, which I was convinced at first was by some unlikely band like Status Quo: I knew it was an instrumental by a band who didn't normally do instrumentals. After trying all my Quo albums (sadly, yes, more than one) I gave up and started going through all my CDs for inspiration. Of course I started at the Z end (closest to my normal chair) so it took a while to reach D. Still, we made it. My CD is a collection of Darkness B sides: this live version is even better (featuring Brian May, no less!) OK, there's not much to the number but a riff, but it's a good riff, and I can confirm that it sticks in the head.