OK, now I feel like an utter banana!
If only I'd listened to your first post! It turns out the solution is a little silly.
The cause was that I had inadvertently activated 3 zones on my keyboard, so every time I hit a key it was playing 3 notes. So I tried your 'play it in on the keyboard and compare' suggestion, and the playback was indeed louder, but I couldn't see any difference between the programmed and played in notes. That is, until I looked in the list view and saw there were 3 notes piled on top of each other.
This is where your comment about round robins comes in. I can only guess that somehow when receiving 3 simultaneous notes, Play stacks the round robins, somewhat like the same option in Storm Choir and that produces what sounds like a higher velocity layer. Once I stripped out the extra notes and reset the zones, everything was back to normal.
So, there are two bright sides - firstly, my problem has gone away and secondly, I now know how to stack round robins in play if I need a thicker louder sound!
Thanks so much for your help!
PC - Windows 7 - i7 2.66 (8c) - 24GB - Raptors/SSDs - RME HDSP 9652 + 2 64bit & 1 32 bit slaves, Cubase 7.5.2 VE Pro 5.1.11849