And I 'third it (I have the Dip Mus too - remember Durham?).
As Cariad says, you go in at whatever level suits you, and the courses don't have to cost a fortune. Most of them, if you pass them, will earn you points which, if you want to, you can use to transfer towards a degree with another university.
Also, because of their reputation and recognition, you can often get an employer to sponsor you on job-related courses.
There are OU students who moan and whinge about certain administrative hiccups, but you get those anywhere (I've also been to conventional university, and it's just the same). Overall experience is good rather than bad.
Have a look at their website and send for a few brochures. Most 'big' courses start in February, so you'd be too late to register for them, but they have short 'taster' courses starting throughout the year that would be a good way of dipping your toes for the first time.
http://www.open.ac.uk/