[SiriusHardware]

Slothie suggested 1K + 220pF so that's what it is at the moment. I stuck an additional capacitor marked '470' across the 220- it's an old British capacitor so the chances are that the value really is 470pF and not 47pF (a more modern 470pF capacitor would be marked '471').

I'm wondering if it would be more productive to knock up a proper monostable delay using a 74121 / 74122 with a multiturn pot to allow free adjustment of the delay in very small increments and generate the delay with that. If that gives us a working delay range we can try to fall back to RC generating the same delay.