2 internal braces fitted - 1 diagonally across the back panel, the other diagonally across the baffle between the port and the driver. A silly idea perhaps, but it seemed to reduce the resonance tapping the panels.
Stuck some random shaped bits of dead sheet to all panels except the baffle. That helped a bit too.
Then painted the inside with PVA and stuck wadding in. Knocking on the cab is noticeably quieter.
Hopefully the sum of my efforts will result in a clearer sounding cab! Will pop the driver in tomorrow evening to find out…