Just to conclude (in case anyone's interested); called plumber who took flue pipe apart. A rubber/silicone seal joining the inner tubes was knackered and a steady, but small amount of steam was leaking out into the casing tube. This steam condensed again, leading to the small but steady drip problem I had.
Plumber replaced the seal and I have had no issues since.
Plumber replaced the seal and I have had no issues since.