Thanks Kramer. I let it run a couple of degrees warmer overnight to try and increase the fermentation activity. The off flavour is far less pronounce this morning, so I am now beginning to suspect that the culprit is yeast. I used 3 grams more years than the recipe called for (23gms instead of 20), and assumed that this would not make much difference, but perhaps it has.
Anyway, I can deal with yeast, it is preferred to mould. I did everything right with this batch so I was surprised to think that I had a mould infection. I'll run it warm for a day or two longer and see what happens.