An update.
On Friday I visited the local college. The computer networking lecturer wasn't there so I spoke to another member of staff who suggested it would be a good idea to learn about general computer hardware before looking at networks.
I mentioned that I was interested in the Microsoft course and he said that although the college didn't offer that course he was familiar with it. He said that the great advantage of the course was that 75% of all commercial hardware in the UK uses MS systems. However, the disadvantage is that the course has been adversely affected in the last 24 to 18 months by cheats who had been cribbing the exam answers off the internet. As a result, Microsoft had clamped down on the exams. He claimed that only 22% of those who sat the exam passed it, and they tended to be people who had studied computer hardware and networks elsewhere.
At that point the computer network lecturer entered the room, so I was passed to him. He explained that whilst some knowledge of computer operation was desirable, it was by no means essential to understanding networks. He suggested that I should study the Network+ certification course (OCR level 3) validated by CompTIA which would cover all networks irrespective of vendor. It's a one year part time course costing more than £350 (the latest costs aren't available yet) but I shouldn't have to pay the full amount because I'm not working. I'm also thinking of doing a couple of computer maintenance courses, just for general background knowledge.
The big problem with studying over a year is that it would delay my entry into the job market by another year. On the other hand, it would give me a more rounded education.
Decisions, decisions...