Why does my mortgage interest rate fluctuate?

[wewereherefirst]

We're on a fixed rate mortgage, for two years but we've noticed in the past few months that the interest raises and lowers each month without rhyme or reason- is this supposed to happen?

we're paying more now than we were at the beginning of the term and I'm not literate in this sort of thing to know if it's right or not.

Are you definitely on a fixed rate?
Contact your bank as the amount on a fixed rate is, well, fixed. That's the point. Perhaps your term ended and you're on the SVR.

SVR hasn't moved for ages either though has it?

Are you on some sort of payment where interest is calculated & added daily and therefore fluctuates with differing number of days per month?

We changed about 18months ago and it was a 2 year fixed deal as we were on a variable rate. I'll ring them this afternoon and see what's what. Thank you!

SVR is set by individual banks, they can do with it as they please.
The bank should be able to send you a statement showing your balance and payments, and explaining any variation.