If you leave the lights on when the engine is off for a few minutes in the dc5, is it supposed to completely drain the battery to a point where the car wont start without a push.
That can happen with any car if the battery is low, not just a DC5.
If it's not been changed and is the original Japanese battery then it could be getting a bit tired. Jap batteries are famous for being weaker. Nothing to worry about though, just get another
The car i looked at I left the lights on for literally a few minutes and the car just died out. I assume that's not normal for a dc5 so could be a weak battery.
Halfords will do one of their own for about £50 which a lot of people use. They list the DC5 in those flick book things in store now too so should be easy to find the right one
Most people do it themselves using the guide on this forum. Not sure how much a garage or bodyshop would charge, it's not a big job though so I'd hope not too much.