You fix one thing, something else breaks!

How about if it was connected to the oil pressure light? Then if pressure drops, the engine stops?

 

Whichever one you use, I'd suggest taking the +12V supply for the pump from the main terminal on the starter (through a fuse). It's only just round the corner from the pump. Don't really want to load up your ignition switch with too much stuff.

 

Right, I'm officially out of my depth and starting to drown!

This is what I've done so far:
1) Power from fuse box (switched live) to illuminated on/off switch
2) Power from illuminated on/off switch to relay terminal 30
3) Original blue wire to battery charge light spliced to relay terminal 86
4) Relay terminal 87 to the fuel pump
5 Relay terminal 85 and earth on illuminated on/off switch taken to good earth
6) No push button connected yet.

Symptoms:
1) Ignition off - fuel pump off, on/off switch does nothing (correct)
2) Ignition on - charge light illuminated (correct)
- switch on - illuminates (correct) but pump starts (bad)
3) Pull relay from socket - battery charge light goes off (dunno)
- pump stops (good/bad/dunno)

To continue the theme of this post, there's also a minor fuel leak developed from the hard piping through the fire wall and the one year old hoses are starting to crack!

If I wasn't so emotionally involved with the van (had it 15 years) but did have agreed value insurance, I'd be tempted to let it keep leaking until it was no longer my problem! AAAAAgggghhhhh!!!

 

It wont work wired like that^.

Do this:
1) Power from fuse box (ignition switched live) to terminal 30 and 85 – you don’t need a separate on/off switch.
2) Blue wire to battery charge light (or low oil pressure light) spliced to relay terminal 86
3) Relay terminal 87a (not 87) to the fuel pump positive+
4) If you want a push button to prime the carbs connect it between 87 and 87a.
5) Pump negative- to a good earth.
Note: Don’t earth 85 – this should be ignition live with 30.

 

That’s brilliant, many thanks for the easy instructions. I think part of the problem has been the two very similar diagrams. I had a feeling I’d gone wrong with not splicing two connections and in Jim’s second diagram there‘s a connection to earth that I definitely didn’t have before.
I’m normally ok with simple electrics but I was going round in circles with this.
Fingers crossed I sort it tomorrow!

 

@Uncle Nick I should have read your first post. ‘So, when I switch the ignition on the oil light comes on but no battery charge light.’

If the battery charge light doesn’t work the relay won’t work. The relay needs to see zero volts at 86 (the blue wire) with the ignition on and about 14v with the engine running and alternator charging.

 

Yes how the charge light works is a bit witchcraft but I think I’ve got my head round it now. It’s wired as per your instructions / Jim’s diagram and working correctly. I think the issue may have been dodgy connections and cleaning them up has fixed it.
I think no 12v signal from the alternator stopped the charge light and relay / fuel pump from working.

A huge thank you to everyone for helping