OK, so its an underpitching issue. Thanks for the explanation Wolfy. Makes me think I might do starters for all my brews from now on rather than sprinkling dry yeast.
No need to make a starter using dry yeast. If the wort is of a gravity that requires more yeast then pitch the appropriate number of packets of dry yeast. I am by no means suggesting that it is not a good idea to proof the yeast by re-hydrating it prior to pitching.