Haha, glad I'm being of use.
Well, the way the degree is structured is that you take 6 modules. 3 of those are 'core' modules, that EVERYONE takes. Then, for your fourth module, you're given a list of certain modules that you MUST choose from. Then, for your final two modules, you can take any module you want from whatever department, as long as you register in time and they let you in. More info here - http://www.dur.ac.uk/resources/faculty.handbook/degrees/frameworks/l100.pdf
Anyway, if you want to beef up the maths (nice phrase! XD ) then you can take a maths module, or an engineering module, or a physics module or whatever in either of your last two slots - or fill both of the slots with maths-y things if you want.
As for time - each economics module has 2 lectures a week and one seminar a fortnight (in general). Hence, if you take all economics/business modules, then you'll end up with 30 hours of contact time per fortnight, which is a lot for an 'Arts' degree. That's not including time spent out of lectures and seminars on seminar prep. and reading, which can chalk up a lot of hours depending on your ability and the complexity of the work. I'll warn you that The World Economy (one of the core modules) is /very/ work intensive - even at A level, you don't encounter a lot of the content, and certainly not to the depth it's studied here.
That said, there is still plenty of time to socialise if you work smart. Only once or twice in the entire first term I didn't go to a social commitment due to work. Also, in terms of exams and essays, it's relatively light on workload, so that's not a problem. Don't worry at all about it, in other words =]
As for Languages, there's a course available called 'Languages For All', which are basically evening classes that aren't included in your degree. For more info, check here - http://www.dur.ac.uk/mlac/cfls/lfa/main/