[Sparkyrog]

Sounds as if it's not running hot enough ,combined with green wood that wood explain it 

[The Silver Surfer]

We had a leakage a month or so ago, it made a right mess.  I couldn't work out why it was doing it.  I then removed the external section of the flue pipe to find a blockage which was severely choking the smoke escaping.  I cleared out the blockage and the dripping stopped.

[Trillium]

It's usually a sign of too much softwood being burned which has a lot of resins and becomes creosote (the black leaky stuff dripping everywhere). Get enough in there and it can ignite itself and become a chimney fire.

A too-tall chimney can cool down faster at the top before it exits the house and can also cause the black runny creosote.

[The Silver Surfer]

Whilst I once again agree with your analysis Trillium, in our case I know it's not too much softwood burning.  Probably 5% is softwood 95% Olive.  However the long run of flue pipe is a probable issue (ours being around 15m or so).  I do believe that the long run coupled with the blockage was the culprit for our leak.  However, now the blockage has been removed everything is fine, and the flue length remains the same.  I agree with your point on length though which has the potential to be an issue.