Regarding which is the outer foil of non-polarised capacitors, if you look at the second picture in post #5 above - the unrestored chassis of my Cossor '501 Melody Maker' - you'll see several waxy paper caps with the outer foil marked with a band at one end. As Sam said, the outer foil should be grounded to prevent hum being induced. I've attached a pic below of the caps that I removed from that set, which shows the bands more clearly. (Incidentally, should anyone wonder why I didn't replace the 25uF 25V cathode bypass cap in the pic of the restored chassis, I did. Modern ones look so tiny and silly that I stuffed the old can).
Though the outer foil isn't marked on modern non-polarised caps, it's quick and easy to check them, either with a scope, or in the absence of a scope, using the input of an amplifier. If you hold the cap between your fingers without touching the wires at either end, your body acts as an antenna and will pick up 50Hz hum. If the cap is then connected to either a scope or the input of an amplifier, hum will be noticed with one end grounded, but not when the other end is. If no hum is observed, the end which is grounded is the outer foil, which should be grounded in the set.
When I buy a batch of caps, I check them all and mark the outer foil with a felt tip pen so when I come to use them, I ground that end. It only takes minutes to check a batch. I wrote it up as a thread a couple of years ago here:
http://www.vintage-radio.net/forum/s...+of+capacitors
The difference is quite marked, as can be seen on the scope traces below. The first trace shows the cap connected to the scope with the outer foil grounded and the cap held between my fingers - as can be seen, the trace is a flat line. On the second trace, the outer foil (as discovered on test) was not grounded, so the cap picked up 50Hz hum induced from my fingers. Confusingly, that particular cap had a black line at each end!
Many may not be aware of this phenomenon or bother to give it a second thought and may even consider it to be borderline OCD. I didn't bother until I became aware of it, and many times it won't cause a problem, but then if it does, we could be on a wild goose chase, changing smoothing/reservoir caps, output valves etc, trying to reduce or eliminate hum.
Hope that's of interest.