You need to enter your original term length, amount and interest rate into a mortgage calculator to check that it comes to the same as what you are paying.
If you are not on a fixed rate, you should be able to swap to another deal with a cheaper interest rate, subject to qualifying for a new deal, if your circumstances have changed, eg lower income, you may no longer qualify - you could be on your lenders SVR, which is often higher, but not always.
Have a look at the moneysavingexpert guide to remortgaging for a walk through guide. You may also benefit from talking to a broker to recommend a new deal - watch out for fees though, which can be high and £120k is a relatively small mortgage, even though it might not feel like it.
It got complicated a few years ago when my last fixed rate ended - when I rang to take out a new fixed rate, the call handler made a gross error and said I wasn't eligible, as my house was in negative equity.
She was totally wrong as it was then and is now at least £60k in equity but she was using some in-house chart and said it cannot be challenged. Since then I was paying over the odds on a SVR.
A couple of years ago, I found out it was an erroneous decision and I could have challenged it.
I had to switch to interest only for a while because it was too high on SVR and it led to arrears as my income dropped - so it is a temp measure to reduce cost while I challenge the erroneous decision , that unfairly stuck me on SVR.
But although i'm challenging the SVR high rate I was paying which led to subsequent arrears, I'm also trying to find out what the average repayments should be; as I've been stuck in SVR so long and interest only, so don't have a clue what the real picture is out there anymore and my mates don't have mortgages (and I don't trust relying on just the lenders info this time).
I'm arguing I've been overcharged thousands due to being unfairly refused a fixed rate by error and wrong info. So, they might write off the arrears, as well as review my repayment rate.
Thanks guys, for your info and advice it has really helped me get a broad picture - I really needed an insight, now i know what to prepare for.
Titchy, good call, I will state any money has to come off the balance, cheers - I did contact the CEO's office and it is finally being properly handled and investigated, before then I was messed about for over a year.
Even if i'm successful with my complaint, there could still be problems - I've got some guidance now thanks to all of you on average repayment rates but I have no idea what revised figure they'll come up with or how affordable it will be.