After reading all of the comments, if the pluges were fouled or excessively worn, not a dealer issue.
I agree with the guys who are pointing out that funning a motor at 3200 rpm, while not planing, is more than likely an excessive load on the motor,
Surtees boats are known for being power hungry, the deep vee hulls dont lift well, somit is quite concievable that this rpm versus speed is correct, if the boat is at the point where a little more throttle would result in it getting over the hump, and planing, with engine revs then rising significantly without any throttle adjustment.
Americans would call it lugging, here, perhaps labouring the motor. Excessive plug wear and deposits on the electrodes will occur. Enrichment of the mixture is very possible.your fuel consumption per km must be horrible. The boat is simply not suitable to be trolling at those speeds. 5.5 to 6kn, the point before the bow lifts is the best speed for the boat, and minimal fuel will be used to achieve it.
Outboards will always chew through plugs far quicker than cars, as cars run very lightly loaded compared to outboards.
The delay in the account being forwarded is very bad business practice.
you can't fix an idiot with duct tape, but it does muffle them for a while...