Dark smoke from gas combi boiler.

A question about : Dark smoke from gas combi boiler.

A neighbour knocked my door this evening to tell me he could see dark smoke rising from my roof. It turned out to be from the boiler flue. It's a Vaillant Ecotec 32. It hasn't had a service for a couple of years, and other than the dark smoke seems to be working normally.
Naturally, I'll phone someone to come out first thing tomorrow, but is it safe to keep using the boiler or should I turn it off straight away?

Best answers:

• Was it dark or daylight when they saw the dark smoke?
• Not being a heating engineer, I thought there might have possibly been fairly common issues with soot/dust/carbon on the burners, hence my question.
• something is burning other than gas I'd think. Anything from collapsed heat pads within the combustion box to a dead bird barbecuing somewhere within the flue/chimney
  We're heading towards the nesting season and birds are looking for suitable accommodation