Bent chainring or cranks?

Could possibly be a tight link in the chain? You haven't got a half link in there have you?

Nope. Bog standard SRAM 1/8" chain. I gave it a proper clean down and then went through every single link with a drop of lube, too. Made no difference at all.

both my bikes have slightly out of whack crank spiders..

one's noisy, one isn't.

I think these things are common with less expensive cranks and rings..

Yeah, it's weird. Slightly disappointing as I spent the extra £50 to get the Miche ones over the Sugino Mess RD thinking I wouldn't get any hassle like this. I just find it really weird it's suddenly started making more noise :-/

obivious but clicking cracking noise on down peddle could just require bit of re-greasing before putting back chainset-happened to me after lots of wet riding days?

Hmm, I don't think it's that because I can see that the chain moves side to side as the cranks rotate. I'll guess I'll find out tomorrow when/if I put them back on.