Benni, Dave, David - Many thanks Gents

I had two days of leave from work; unfortunately not for shop, but to attend to some matters relating to government red tape...
At least I bought a good quality pressure gauge yesterday - at very good discount from one of my regular suppliers. I'm finding that a lot of the suppliers that up to a year or so ago treated me a bit reservedly has nicely thawed; where initially discount and a bit of natter was taboo, that's run of the mill now. Got the gauge for less than half the shelf price!
This afternoon I got a bit of shop time.
Started by turning down and single-point threading a bit of 8mm hex brass to the same dimensions and thread as the gas filler valve:

Then parted off a section, flipped it in the chuck, drilled through at 2mm and then drilled and tapped it M6x0.5 to fit existing adapters for my pressure testing kit. Ended up with this:

It needed an O ring - so I added that:

Then I set up for testing - replacing my 1000kpa gauge with the new 4000kpa gauge. The gas tank was filled with water - though not completely. It is so small that a bit of air in it is needed to allow at least some form of control from the pump; if there is a leak it won't be too much:

On the first pressure-up I took it to 3000kpa, but the pressure dropped rapidly. On checking, I found water leaking around the valve spindle packing, so I just snuck up the packing nut a bit more.
Next pressure up:

I left it sitting for 20 minutes, and came back to find the pressure had dropped slightly:

The gas tank was bone dry, so I looked around to find the leak; i eventually found it on the regulator adapter where a droplet of water had formed on the connection - but all-in-all, I think it's safe to say the gas tank is up to scratch

As a final test, I screwed in the gas filler and filled the tank with gas, then plonked it in the water to see if there was any gas escaping by looking for bubbles:

No bubbles appeared... I also tested the regulator while under water; it is easy to control and get varying amounts of gas escaping.
Having done that lot, I treated myself

:

, Arnold