for all inset coal type fires, you MUST always replace the coals with the correct ones for that make and model of fire, you never ever add extra coals or use ones not intended for that fire. And yes the coals have to be arranged as per the manufacturers instructions otherwise the fire may not burn properly and produce Carbon Monoxide.
Try ringing Gas-on-spares of Halesowen, if they can't help you no-one can. For a set of coals and the fuel bed tho expect to pay around £75-£100.
Gas-on-spares may require the G.C number of the fire, this is normally found on a large metal data badge which is hidden underneath the burner and usually chained to the metal fire-box.
gas-on-spares are on 0800 389 0907 and they also have a website
http://www.gasonspares.co.uk/