Just be aware that you will be alongside people who have done a 3 year degree in that subject and know a great deal more about it than than you do. Not just the subject but also the theory behind it. A Masters is designed to build on this existing knowledge and take you into more specialist areas, potentially as a preparation for a PhD.
However, it IS do-able. If the two subjects are allied/related then, whilst you will have to get up to speed very quickly on the basics undergrad stuff, then it isnt too much of a climb. Your application would be looked at in that light, and if you can show in your PS that a) you understand the challenge, and b) have the comprehension of the 'new' subject to cope, then you should be okay.
Two final points - why do you want to do a Masters? You need to be very honest with yourself about this question. Many people do a Masters because they think it'll bump them up the job queue - it usually doesn't, because unless you have relevant work experience, you will still be a 'new graduate with no experience'. Also, remember the cost. Aside from the £9,000 loan, there is no guaranteed funding. How will you fund this and will it be worth it if it doesn't necessarily make any difference to your job prospects?
I'd say 'Apply' - but think carefully first.