Thank you very much for your detailed responses.....
The deck of the board between the bolt holes looks fine, there are no cracks or other signs of damage. Inside the bolt holes (from the deck side) there also looks like there is no damage but the finish is kind of rough given the finish of the rest of the board.
There are no cracks (but lots of scratches) that I can see on the side walls of the fin box (looking from the bottom of the board). There is one crack the full distance of the front section of the fin box and one crack running the full distance of the back section of the fin box, the crack on the back section is worse than that at the front section and from the distance the back section crack runs onto the bottom of the board it looks to be about 3mm deep.
I will try the soap test and see how I go from there....
I appreciate the time you have taken to provide such detailed advice, thank you!