Don't Siemens do any kind of sponsorship or Education Assistance/Day Release for appropriate qualifications? A lot of modern companies tend to nowadays, I know mine does. This way you can keep accruing your experience (and your position!) while gaining complementary quallies.
Failing that, as others have said, Open University. Check with your boss whether they might help towards it. Remember though, it must be something relevant to your job. I don't think they'd be to warm to you doing a degree in Klingon
HTH