Hello Chris,
This is obviously a battery or charging system fault , and if you have a multi meter and are handy with it then you can fairly easily find out which. I doubt if your battery is the original after 12 years, but it is possible. If it is, then you have done well, and it would be more likely to be this that is at fault - but you will still need to confirm the alternator output.
I would suspect that this car now has insufficient battery power to start the car, but if it does start, a fairly simply test for basic charging is to allow the car to tick over for a few minutes then switch the headlights on, then observe if the headlight brightness increases as you rev the engine. No increase in brightness suggests a charging issue.
You should find that Halfords or others will be able to check your charging system quite simply -while you wait . If OK and the battety is a fault then buying a replacement at Halfords?? If it were mine, I would find a trusted local auto electrician and get them to check this out for you.
One other basic test you should do, is to check the serviceability of the alternator belt. This may, just may be the cause of the problem, but should be eliminated as the culprit. Suspect alternator belts giving way, wrapping themselves around the crank pulley and causing cam belts to slip are not unheard of.
If the car won't start and you need to jump start it then please make sure you know what you are doing before connecting jump leads or you can cause all sorts of expensive problems.
Kind regards,
Gareth.