There's one compulsory maths/stats module in 1st year, one compulsory stats module in second year and an optional maths module in second year. There's also an optional 'applied econometrics' module in 3rd year.
In the second year microeconomics and macroeconomics modules you'll have to use maths to answer a lot of the questions, but it's mainly differentiation.
All the modules are 20 credits, so you take 6 modules per year.
I think you're allowed to take up to 2 maths modules from the maths department in 1st year and one in second year, but you'd have to check as you're not supposed to take some maths modules with the first year maths for economics module.
You can take a finance module in 2nd year, and a financial theory and security investment analysis modules in 3rd year, but they aren't maths modules.
You can see the module list here: http://www.dur.ac.uk/resources/faculty.handbook/degrees/frameworks/l100.pdf
It's probably best if you email the department and ask them, they'll be able to give you a clear answer.
If you really want to study maths, you should probably ask if you can transfer your application to the BSc Natural Sciences joint honours degree in maths and economics.http://www.dur.ac.uk/natural.sciences/prospective/