Following my carb rebuild and subsequent petcock issues I have rebuilt and de-vacced the original petcock and just in case anyone finds this useful - and as a thank you for the help you gave me - I have included the pictures.
This is the kit that comes to replace the rubber. I got it from a local carb guy and it cost me about $11 (£9). It also includes 2 X 3mm bolts. I have no idea what the middle sized o ring is for.
Grind off the rivets and then drill them out.
You are supposed to do this with a 2.5mm drill bit and then tap the holes with a 3mm tap. Unfortunately i didn't have a 3mm tap so I drilled them out and then used a small 4mm nut and bolt when it came to putting it back together.
This is what you get when you open up the petcock. You can see an O ring to the top of the petcock and there is one below the ball bearing the same. These 2 rings are actually joined together with a figure 8 type arrangement (shown in later pic) but they split quite easily. I removed the first one with a small cutting blade then hooked a screwdriver round it and pulled it out. The second one came out easily. The replacements are the 2 thinner rings on the original photo. The seal around the petcock comes out with a sharp knife, and that is replaced by the large O ring in the top picture.
This is the petcock with the 2 new O rings in place and the old ones for reference. I haven't put the big sealing ring on yet.
The selector then pops out of the face plate and there are 2 thicker O rings on there which come off and are replaced with the 2 thicker rings in the top picure. It then goes back in to the face place taking care not to drop the plastic washer above the O rings.
This is the selector with the rings removed, next to the figure 8 ring from earlier.
We then move on to the vacuum side of the petcock. Having removed the 4 screws and all the innards and the dividing plate with the vent hole in it, I JB welded a Polish 20 zloty coin over the hole. It will work with other european coins and possibly some of your currency as well.
This has now turned the tap into a straight on/off/reserve with no vac section to leak back into my cylinder number 6 and cause the hydrolock that occured previously.
In addition to this I have installed a 12v fuel shut off solenoid, just in case I forget to turn off my tap, and the needle valves did fail. This has a little manual override in case there is any electrical issues to get me home. I used this one from Italy because I am in the UK
https://www.tomasetto.com/en/products/accessories/solenoid-valves/elettrovalvola-benzina-2 and it cost me about £25 (£20) delivered next day.
