A number of dark spots and bigger patches have appeared on many of the large grey limestone slabs that comprise my patio. It looked beautiful a few years ago when newly laid, but is now quite badly disfigured by them.

I've tried using a pressure watcher at close quarters on its highest setting, and scrubbing at them with sodium bicarbonate and also with Karcher tone/patio cleaner, but nothing seems to shift them.

Is there a product that will remove them?

Possibly algae is the problem.

Are you sure it is limestone and not slate? Doesn't really matter. I have used a power washer cleaner product from Home Depot and Lowe's. That worked pretty well.
Marble and slate are not very porous if at all. Is the discoloration in the grout or cement filled cracks? Both of those are porous and will stain. In that case, try a 10% house hold bleach solution. Let it sit for a bit and then scrub and rinse. Don't use too much bleach because that could cause other problems.

Can you post some photos? Where is 'Bucks'?

Buck's County I assume.

From the spellings I recon it is Buckinghamshire UK.

Iochroma said: From the spellings I recon it is Buckinghamshire UK.

That would be my guess too, also based on the spelling of "mould".

It could very well be. That's for sure.
My advice would not change.
Hopefully the person gets his patio cleaned to their liking.

I am possibly the worlds worse speller! Some days it seems like I can't spell my own name.

Nothing incorrectly spelled here, Americans spell it as mold while the British spell it mould.