On the bottom of each page of a "requested" mortgage statement are the words, " The Borrower shown on this statement remains responsible for all sums outstanding, including those not recorded on this statement due to error or omission" Does this mean they can charge just about anything, purely by saying, "we omitted it in the 1st place"?
No. It means that if they have made a genuine error or omission then you are still liable for any sums which are not on the statement - provided they are sums which you legally owe under the terms of the mortgage.