Hi, Vasquoy
I don't know if you can buy a type of small coal called 'slack'. What I used to do was put slack in a plastic bag and wait until the fire was burning well, then put it on top of the burning coals towards the back of the fire. I made sure the dampers were closed off so that there was no draught, but if the fire seemed a bit dull, I gave it a bit of a rake (maybe opened the dampers a bit).
At night, I did the same thing, but dampened the slack first and put enough in the bag to cover the fire well. I raked it in the morning - hey presto! If you try this, don't forget the fireguard.
Any environmemtal campaigners out there - don't pick holes in what I did - I just wanted to stay warm in an old, cold house with no CH.