I had the same issue with a buy to let Ulster Bank mortgage, had tracker "for the lifetime of the mortgage", fixed twice between 2007-2010 and was put back on a variable rate following the expiration of both fixed terms. I was within the 6 year limit when I discovered the issue (only just) but I elected to seek proper advice from a professional on getting my tracker back. I used Paraic Kissane (goes by the name of PadKiss here). I was successful in getting my tracker restored and all over payments were refunded to me, the whole process took 5 months with no involvement with the Ombudsman. Obviously there's professional fees to be paid for this but I wasn't willing to run the risk of the Ombudsman ruling against me, as he's done in some cases.