It seems big companies throw them out when the batteries fail, and here, batteries tailored for this unit are about $20 each on't net. Not much more from APC. Hopefully, someone in the UK would be able to offer such prices if you happen across anything used.
I have a SmartUPS 1000 - APC branded replacement batteries are £162 all-in, 3rd-party eqiuvalent replacements are £62. Hmmm... for the effort of swapping over the spade connectors I can save £100!
Batteries on the business-grade units (i.e. Smart-Ups / Smart-Ups online) are replaceable and replacements are easily obtained from APC.
However should you feel so inclined, you can usually do a trade-in deal with APC so you get a brand new unit and they take care of disposing of your existing unit in an environmentally sound manner.
The trade-in for an equivalent model would reduce the price by £71 to £278, although you can buy that model for £30 less on amazon (granted they won't take the old unit away at no cost).
I'll be buying 3rd party batteries and carrying the old ones to the dump (sorry, recycling centre) on my bike!
SD