the sound of water boiling and steam in attic

Hi I have a 30 year old oil boiler and a back boiler with a pump also. Last night I had a big fire on and had pump on but I could hear the sound of boiling water in the pipes near fire place. I took a look in attic and the small tank which had the water for the heating system was bubbling and steam rising and there was water around it as if the tank was boiling over. This water is very dirty. I turned on the hot water and let it run. I turned off everything and let the fire go down.
I will be calling a plumber today. Is it that the pump was not working ? Is is something else? If the plumber does not come today will it be safe to turn on heat or light a small fire.
Thanks in advance for advice.

I am not a plumber, I would be inclined to put on the central heating tonight but not light a fire until you get the pump checked out. Maybe you shoul ring the plumber and ask him to confirm this.

Also not a plumber.

That sounds like an expansion tank for the central heating. If the pumps weren't working you'd have had no hot radiators. Running the hot water would have no effect as its a different loop. Sounds like your Central heating is over heating. So running at it lower temps or for less time should temporarily help the problem. Sounds like you have a thermostat somewhere that's not working or the system needs cleaning out, and perhaps also theres a pump not running 100%. The dirty water suggests you're CH system need to be flushed.

But you need to get a plumber who specialises in central heating to check the whole system.

http://www.diynot.com/forums/viewtopic.php?t=10193

You have known as 'kettled' the system.
The heat in the water has nowhere to go so it boiled.
As previously said, you have a high chance the pump has stopped working.
If the back boiler pipe circuit runs through the cylinder, running the hot water removes heat from the cylinder and allows the back boiler circuit to dump heat into the cylinder. This will cool the system.
Lighting a small fire for a short while won't repeat the first event.
Time for an overhaul of your system..