Its unlikely you will be able to get replacement parts for the fire as Potterton have outsourced the manufacture of parts 2-3 times in the last 15 years alone, that said there maybe generic coals which can be used.
The serial number isnt coming up in the Potterton database, on the data plaque, is there a Gas Council code, or model number shown?
Ok, so we need to know which of the three you have, attached are images of the three models, appreciate Knighstbridge and Presence 2 look the same, the key difference is the Presence 2 hasnt got a bevelled edge around the outside.
However from taking a brief look I think you may struggle to find replacement coals, but will only know for sure once I go back with exact model \ gas code to the suppliers.
I have spoken to Potterton technical services, they have identified the part numbers apparently the coals are numbered for a complete set you need 360/2040, 360/2041, 360/2042, 360/2043, 360/2044, 360/2045, 360/2046, and 360/2047.
They have suggested as the parts appear to be obsolete, if they arent cracked\damaged and just lost there colour your best bet is to spray them with something like the product here, which you can pick up from your local plumb centre.
So unfortunately though I have managed to trace down the part numbers, it doesnt look like you will be able to get new coals.
Please let me know if you need anything clarified.
From talking to the technical team in Potterton who have been very helpful, the coals are purpose made for that fire, some fires you can use generic coals so even if you found another Potterton you would probably find they wouldnt fit, and cause issues when it came to fitting them due to the amount of CO2 given off, or overheating etc.
For a high energy appliance, which has fire safety consequences I think you will struggle to find anything which would give you peace of mind.
However in terms of after market, you could try some of your yellow pages for local firms who repair gas fires in your area, as the engineers may collect dead \ end of life units, and do them up. I have all sorts of valves and elements I have collected over the years so there maybe someone locally who has what you need, but dont know of any online aftermarket re-sellers.