Sorted now. 'part 38' was slightly lose.

It seems a bit of an odd design, but the valve block, part 38 and the toast rack form a chamber, between the regulator and the knock off valve, but there are no seals between them, just one bolt through the middle.

There's no o-ring or gaskets, just a surface seal. So it doesn't take much for the air to leak out...

For some reason, taking the bottle off and bleeding off the regulator chamber allowed it to relax a bit and leak.

When I say lose, it needed maybe a 1/8 turn.

There's a vid on XTX showing the tightening of part 38 and it says it can become lose in transit or after taking off the bottle!