"Are there grounds for complaint here - either to/about the broker, the ultiamte mortgage provider and - eventually if necessary - to the FSO? While the terms & conditions probably provide for such last minute rate changes it doesn't seem fair for the borrower not to be even notified. Personally I would kick off a complaints process with the broker and/or lender and take it to the FSO if necessary."
To be fair - fixed term offers are always stated to be fixed as at date of drawdown. Given the fact that there have been several rate changes, I don't really see the issue.
And from OP it does seem as if he was notified of rate change possibly before drawdown thereby allowing him not to proceed if he so wished.
mf