@Cuppa - wow! Thanks mate. That is a great explanation and gives me something to work on.

I haven't yet measured and compared the output of the panels and will have to replace my multi-meter first (cheap nasty one fell apart yesterday). The wiring may also be on the small side and the panel with the 'worming' is the second panel of the two in the string. The cells that are damaged are the ones furthest away from the controller also.

I have no idea what a 'blocking diode' is, but no doubt my local Jaycar guy will.

By the sounds of it, this sort of thing could happen to any and all solar panels without the blocking diodes? eg. an expensive panel might also have the same problem as these eBay ones.