If you suspect that your mortgage repayments are incorrect then you should indeed contact your lender. If they are actually lower than they should be then you will end up paying more in interest and/or a longer term in the end. If you can post more details (in particular your mortgage amount, term and APR rate) then people might be able to estimate what you should be paying. Also - if you are buying mortgage protection life assurance and/or home insurance from the lender and this is included in the figures above then you should post details of the premiums.
You should contact the lender and get this sorted, as it will effect your credit rating in the future, as it will show up mortgage arrears and no one will give you a loan etc based on this information,
As Clubman suggests, get this sorted straight away! Ultimately you will be more out of pocket if you allow this to continue. My own lender made an error in deducting my mortgage payment over a short period but I took them to task over it and I now overpay to shorten the term of the mortgage.
wouldn't be ulster bank by any chance? I noticed the same thing last month but seeing as I'm only paying the mortgage a couple of months I'll see what happens next month...after Christmas and then look into it.