It's funny that you mention that a cleaning may bring back the tube...because two things occured to me.
1). If it is a sealed system then the process of generating (exciting the gas) the light must be modifing/degrading the gas. So be it - nothing is free.

2). I used a tube that I took out of another unit (I changed it because it too was bad and had not yet thrown it away ) and it worked. Probably the very fact of changing the tube out caused a type of scoring of the contacts and inadvertantly cleaned them.

2a). Taking Erik's (and your's) suggestion, I am not going to discard the old tube. It was a real eye-opener!

I have an old MG Midget (1974)...I think I'll take a look at those points when summer returns. :)

Carl