Thanks for the reply Radu (& the kind consolation!),

I had the problem before I tried installing the middle clip; that’s why I installed it, because I thought it might help.
The middle clip did not ‘appear’ to actually puncture the tube, it just crushed it inwards (like a soda can).

Do you think that the 2.2K resistor (part R7) could have damaged the tube in the first place? I installed it before realizing it didn’t belong, so the first time the circuit was energized it may have sent an incorrect charge to the tube?

The seller I bought from stated that they have been tested. They are new old stock, & appear visually identical to the ones supplied in the kit originally. I will just wait & see how it goes in a couple of weeks.