Neon Indicator lamp replacement

[dzhangg]

I have this neon indicator lamp that I would like to replace with an LED. It is a part of an old flash gun.
The indicator is made to show "charge complete" and I've measured that to start from 240V and keep going until maybe 350V. The indicator lamp is connected in series with a 1.5Gohm resistor and the whole thing is connected across the charged capacitor.

There's 4AA voltage within the pack which I could use, but i use rechargeable batteries, so it should work down to 4.8V

What's the smallest circuit I could get to replace this with an LED?

 

[Asymptotic]

There are too many ways the 'ready' lamp circuit may have been designed.

A good reference resource for all things strobe flash is
http://repairfaq.cis.upenn.edu/sam/strbfaq.htm