Beaker wrote
I'm pretty sure it works the other way round - polarised electrolytics are "specified", not because they need to be polarised, rather the polarity is shown simply because it is hard to find non-polarised capacitors in values of over 1uF.
That's correct. You can
always replace polarized caps with non-polarized ones, but not the other way around. In other words, if a schematic calls for polarized caps, you can use polarized or non-polarized. But if the schematic calls for non-polarized, then you can't used polarized caps. Polarized caps are never used because they are necessary, only because they are available. Polarity isn't a feature, it's a weakness.
The issue is this: Polarized caps resist DC in one direction, but they let DC pass in the other direction. If you put a polarized cap in backwards, the DC current damages the cap and it will fail, usually closed (ie it will act like a jumper). This will be especially bad if it is a filter cap on the V+ rail because it could burn out your power supply. If you accidentally install a cap backwards and supply power to the circuit, even for just a minute, take out that cap, throw it away, and replace it. It
will fail, probably in the middle of a gig.
Non-polar caps are required when there is no DC bias across the cap. For a very simple example, say your entire pedal consisted of just one cap and no other components. Then your input and output would both be centered around zero volts, and would have both upward and downward voltage swings. So the cap would be reverse-biased half the time. Using a polarized cap here would be a bad idea. It wouldn't fail as quickly as it would if it had DC bias across it, but I wouldn't risk it.
You can make non-polarized caps out of polarized caps in series (C = C1*C2/(C1+C2)). Just make sure the polarities are opposite each other (either the cathodes are touching each other, or the anodes are touching each other). That way, each cap blocks DC in one direction.
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maybe eBay doesn't love me enough yet 
