Couchycool - I think that you need to get your head around what sort of engineering interests you.
The career route and qualifications into design, and the same for maintenance, or to a large extent the same into building new aircraft and equipment, are three quite different routes.
The fellows above are right - line maintenance is pretty tough on the system, and possibly if you've not had a prior life in something physically similar (and intellectually, these chaps use their brains as much as their hands, however much they like to deny it) you'll find changing from a "simpler" lifestyle into that sort of job pretty difficult. I'm not saying you can't, just that it'll be difficult.
If you want to go into the design side, then HNC/degree is the obvious starting route. You then, ideally with an MEng, can either go into one of the companies - most usually through a graduate training scheme (although they'll be a little surprised at applicants in their 40s and that may cause you problems), or do another year somewhere like Cranfield to do a specialist MSc and then slot more quickly into the working environment.
Another route is into the research side (I made this change mid-career from originally working on the design and certification side) by doing a PhD and then looking towards postdoc or lecturer level jobs in universities. Age will probably count least against you here, as universities generally couldn't give a stuff about age, sex, nationality, ethnicity, health - in fact anything but your academic ability. I'd love to say that the rest of industry was like that too, but frankly it's not.
If you have a fascination with aeronautical engineering, and a history of ill health, then certainly the degree rather than maintenance route is likely to favour you more.
Do you have a " a reasonable chance of securing a job" ? Hard to say - generally speaking, in the graduate side, the issue for everybody is getting their first proper graduate job, as employers value experience much more than the academic qualifications alone. Once you are around CEng or postdoc level (depending upon route) then there's a growing shortage of people, so you're unlikely to every be unemployed for long.
G