I've realised my error that caused it to blow...
The switch box I built worked fine for weeks, but I needed to see
visually which channel was on and the status of the reverb, as it's not
always easy to tell when you have two saxophones blasting in your ears
on stage ! Sooo... I added a couple of LEDs switched by the other
poles of the DPDT footswitches and powered from the 9v supply I use for
my effects pedals. I used a 2.1mm connector, metal body, mounted in
the metal footswitch case. Effects pedals are almost always 9v DC,
negative tip. The shield of the 1/4" socket on the footswitch (that
connects to ground on the amp via the cable) was connected to the
footswitch case too - via a metal 1/4" jack.

This meant that as the power is negative tip, the footswitch case was
now 9V positive, and 9v went up the cable to the shield of the jack
socket on the amp - then also to the ring and the tip as I pressed the
footswitches. Without the guitar signal cable connected to the amp, it
worked fine - BUT... when I plugged that in (from the effects pedals) -
I had a direct short, as the shield of the guitar cable is connected to
the negative pole of the 9V psu, but I was sending 9V positive to the
ground of the channel switch jack !

....keeping my fingers crossed it's just the zener !

Thanks for everyone's input on this so far. It's much appreciated.

Cheers,

Kev.