I may well be missing something here (in particular, whether any relevant transaction other than your later purchase of shares for an ISA was other than movement of cash). However, if not ...
- (1) I understand from what others have said you can reverse the current transactions within a time limit, and
(2) You could then redo them in the originally intended way, but
(3) The problem is you have bought shares with the current proceeds
If broadly accurate, then a possible solution is
- (4) II loan you £51K
(5) You reverse the transaction with the loaned £51K
(6) You replicate the equivalent transaction in the originally intended way
(7) You repay the loan to II with the proceeds
I imagine the difficult thing would be to convince II to "loan" you the money, especially if it has to leave one of their accounts and into your own (not sure if this is the case, but am guessing so). However
- (8) If that have equivalent/excess collateral with II, e.g. your ISA, and
(9) You approach them with a constructive attitude that you prefer this collaborative route to a solution, rather than making a formal complaint, then
(10) You never know - don't ask, don't get
Once again, sorry if the above is completely naïve, but if not, it would appear to address the known difficulties.
Regards, Newroad