I saw the frying pan thing suggested a few different places. I think the idea was just too loosen anything that may be contaminating the surface, but just on a low heat to avoid damaging the pad.

The pads weren't old or cracked or they weren't due for replacement, they'd hardly been used. That's why I was reluctant to buy new ones. I just guessed that some dirt/dust had found it's way on to them as they'd been sat unused for a couple of years.

If there's nothing else that can be done to renew them though, I'll just look at replacing them. I don't know the exact model, but they're Shimano hydraulic disc brakes.