I think that I have found a resolution. I worked out the weight of 8 divers plus a driver and it was right on the limit of the published load for the boat. The time that I tested the boat was in fresh (calm) water and over short distances and I've come to a few conclusions :
1 - The boat was slightly overloaded !
2 - Travelling such a short distance with not much chance to build up speed, the bailers weren't getting chance to empty the hull fully.
3 - It's harder to get a boat (on the limit) to plane in flat calm water.
All this said, I took 7 divers + kit + driver out in Dublin bay on Sunday with 2-4 ft seas and the boat was doing 26kts. As I opened the throttle the engine revved to 4k and then as the bailers started to do their job the revs picked up all the way to the 6k limit :-)
FYI. The console is about 1/2 way down the boat and all the tanks are in a rack that stretches from just forward of the transom to the back of the console. All the weight belts and 'bits' were at the bow and the divers sat on the tubes about where the cockpit is.