if you are working full time COLU do the HNC courses on distance learning, I would have thought it would take a lot of discipline to complete as you are expected to manage your time and hand in assignments on time etc with very little input from tutors etc (have done distance learning not HNC but found it very difficult)
If you can spare one day a week then HNC is avaliable on day release for two years and most unis and colleges offereing higher courses offer them in a range of subjects
If you can manage full time then the HND can be taken on that includes the HNC (HND is just extra units) I think this is two years also(?)
Obviously uni may also be an option, not sure about your past qualifications but a degree may be possibkle.
Remember if you complete a HNC then you can transfer a number of points towards an open university degree as I believe it counts as the first year towards it
As for subjects the world is your oyster, you could go down the route of electrical and electronic engineering if you are pursuing a career on the electrical side, mechanical engineering would effectively make you multiskilled although I believe most employers would pretty much disregrard your (i assume you have...) 2360,2330 etc and class you solely as a mechanical engineer.
If you like the thought of diversifying like I did the Mechatronics route is ideal, I was qualified to electrician level but wanted to keep my options open so opted for this course (actually named process engineering, instrumentation and control) it covers maths and business etc but also covers PLC's robotics, 3d CAD design, hydraulics/pneumatics, plant and process principles (basically mech engineering stuff), electronics, electrical power distribution, electrical and mechanical science etc etc
(all depending on your college and tutors etc) I found it hard to find a college offering a tutor in plcs and robotics although bridgwater offer this.
Since competing I have found I can apply to loads of different fields including the design engineer route, biomedical route, control engineer route and electrical and mechanical routes.
As my employment has been electro-mechanical engineer in factories it will only help in my progression to more interesting work ( ie robotics a passion of mine)
good luck mate any help i can be let me know (btw how is your maths
)
Craig