It would probably be better to buy a new one...if it is a small 600VA UPS that is..A new battery costs around R150 and then there would be labour, so it would depend on who you had fit it. A new UPS costs around R400 - R500, but at least it is new.
They usually use the same battery as automatic gates do..12v 7Amp Hour.
IMO you should just swap the battery - very easy to do and hassle-free. How old is the UPS? If older than 3-4 years, then rather replace the unit for an updated one...
Thanks for the advice guys. I think to just save me some hassel I will just get a new ups, the one I have now is 4 years old just a run of the mill proline 650va but I think even if I put a new battery in, it wouldnt help because I have a 600watt psu and a 23 inch lcd hooked up to it. so maybe its time for a more beefy Ups.