Bill, as I see it, there are some words within Haven's reply that give you some leeway.
1. They talk about whether the new incident is linked to the original repairs, or if it is a completely new incident. So, does the position of the current bend in the rudder stock coincide with the original bend? This would not be entirely conclusive, but would lend credence to the original bend and subsequent straightening having weakened the rudder stock.
2. "Check rudder stock has not been damaged." Presumably Mainsail Marine would have produced a written report of their findings which hopefully they sent to Haven along with their proposed repair strategy. It would be useful to get a copy of that report, along with any reply to it from Haven. In particular is whether there was any intention or requirement for a detailed investigation of the metallurgical affect that straightening would have on the strength of the rudder stock, and also if there were any weasel words that would enable Haven to pin the blame on Mainsail. But keep in mind that it would also have been negligent for Haven not to have specified adequately about the residual strength of the rudder stock, particularly as it was straightened at Haven's insistence instead of replacement which hopefully was a request you made to them in writing.
3. On the 26th of June 2014 you said "The repairs so far" etc "have been entirely satisfactory." As I read that, what you have indicated is that up until that date, just four months after Mainsail submitted their estimate, and therefore presumably a lesser time between the actual date of the repair and the date of your statement, that the repairs appear ok. This short period of time cannot be considered as anything other than what you have said, namely up until that moment things look ok, but that cannot be assumed to be a guarantee from you that all really is well, particularly if you are not qualified in regard to the residual strength of the rudder stock. As for the residual strength of the rudder stock, unless they know otherwise I'd say you need to lay it on thick that you are not and were not qualified to make any definitive comment in that respect.
4. Haven say that you had legal representation in effect at that time which you can use, so go and grab it with both hands, but make sure that the legal firm will provide totally unbiased help. It could be worthwhile checking your household insurance to see whether you are also covered under that policy for legal help. It will not be to Haven's advantage for this matter to go to court because you live and presumably the boat was located in Scotland where separate laws apply from the rest of the U.K.
5. Haven did not appoint a surveyor, and they admit that they would not have paid for one even if you had asked for one despite that part of the damage to your boat had the potential to affect its seaworthiness. In that respect I would suggest that they were negligent not only in that respect, but also in respect of the fact that they authorised the work that Mainsail did, and it would seem, they did not seek unbiased assurance from someone qualified to give it that the work had been fully and properly carried out.
As for "pigs might fly," if you don't ask, you don't get, so good luck, you will need it.
P.S. I should have the name of a decent loss adjuster who could help, and if I can find it I will let you know. He comes from Kent, but may be able to advise on an alternative person that lives closer to you and would cost less for them to travel to you.
