If the disc has a thickness variation, it will keep tapping the pads away. If they are a bit stiff in their housing, it will take a while to push them back. If they are not used, they will eventually stay just far enough away to be quiet. Applying the brakes may well bring the noise back again, but once warm, they may move more easily.

There could be another issue, but start here. I have to clean mine out annually.

Ok, next update here.

I managed to free the pads from the disc which allowed the wheels to turn much more smoothly. So then I went about cleaning up the pads and calipers, which went well. One thing I did notice is that when swinging the calipers back down, there was some springy resistance on the nearside one when trying to put the bottom pin back in, however the offside caliper did not have anywhere near as much resistance, could this be of any significance? I took the Panda for a quick run, pumping the brakes before setting off, and there was no knocking or thudding at all. Whilst driving it I found a place to do a some full lock, slow speed turning in each direction, just to make sure CV joints were ok, and they were fine, no knocking sounds at all.

Early indications seem that the problem has been rectified, however the real test will be when the car has been sat for a while, and in dry weather. Only then can I be sure if this work has cured the knocking. I think it'll be quite dry from now until tomorrow morning, and colder, so I'll give it a try in the morning and see what happens.

I'll update this tomorrow to let you know my findings, I've got my fingers crossed!!

Well done, a good result.
Resistance when pushing the caliper down is due to the spring plate inside the caliper, meant to keep the pads from rattling. Presumably one is weaker than the other. Replace when the pads are next done.

Cheers for all your help portland_bill, it looks like your expert knowledge has cured this knocking noise. Its been pretty dry and cold since yesterday evening, so I drove my Panda just a few minutes ago, and there was no knocking at all, quiet as a mouse! Well chuffed!!

I'll keep an ear out for the knocking over the next few days, just to make sure it doesn't come back, but its looking good so far! If it comes back I'll let you all know.

Thanks again for your help, I really do appreciate it.

Just a thought.

Could there be a wheel bearing problem that's aggravated when the brake pads are not retracting?

Driven my Panda loads since cleaning up the brakes, and still no knocking noise happening.

Looks like it's all fixed!

to all for your help and advice with this!