I've spent the last 2 years doing an applied maths Master's part-time, while having a full-on day job.
Assuming you left education a few years ago, you're likely to suddenly realise quite how underused your brain may have become, but if you're pondering a return to university, you will probably enjoy this experience - I found it rather life-affirming.
I would strongly encourage you to have some form of face-to-face contact with lecturers or whoever is running classes - it makes it much easier to understand the subject and pick up any misunderstandings. Additionally if you have a full-time job, finding the hours every week to get through all the material can be a bit of a struggle - lectures do not pause for a week if you have a busy weekend with your family to allow you to catch up.
If you think you can find the time, and really do want to do it, I recommend the experience.
If you are thinking of a Masters rather than Bachelors, you might like to have a look at the RAE scores published last year for each subject at each university. They're done every 4 years or so and are the ones that determine how much funding each unievrsity department gets. Forget the surveys in the newspapers - the RAE is the ranking that each university really cares about.
RAE 2008
RAE 2008 : Quality profiles
Send me a private message if you would like to discuss further