Hi RoC,
I don't think that you've left it too late to consider a career in networking. I've always found that the networks gurus that I've worked with have always been older than your average IT person. I put this down to kids at home not having access to a bunch of Cisco routers and switches, so not being able to practice and play about with stuff, in the way that they can do with programming, sys admin stuff, etc... As such, people don't tend to get exposed to networks until a later age, and this is reflected in the average age of your network specialist.
I guess things might have changed slightly as more and more people build their own LANs at home to hook up 2 or 3 PCs and maybe add a wireless AP, but this is still small fry compared to the sort of stuff they'd teach you on a qualified course in preparation for building corporate-strength n-tier networks.
I'm not a networks specialist, but I would personally stay away from those companies you see advertised on the TV, that promise you a guaranteed job with a £30K salary and a company car and a new girlfriend as soon as you've finished their 6-week course.
That's not to say that being CCNA (Cisco Certified Network Associate) certified wouldn't stand you in good stead, as it definitely would, but choose your qualifications/certifications wisely as they can be horribly expensive if you go for the wrong one.
I would think that the HNC that you did before would be a good move, but I'm sure one of the other peeps on here whose job is in networks will be able to point you in the right direction.
I would also suggest that you be prepared to take a junior position to start with, as you'll most likely find that you're going to learn just as much (if not more) when you're on-the-job, as opposed to in a classroom.
Good luck, hope it works out for you