Back to topic.
I have been given yet further information which has shed more light on the subject. It would seem that the information I was originally given was in fact very slightly off the mark.
I can now state with some confidence that we would be in receipt of 25% of anything over and above the original £150k.
Better get that humble pie in the oven, then!
Please don't shoot the messenger, though. I was acting in good faith.