Takes mod hat off:-
franko, they aren't too expensive if you go to South Yorkshire Battery Supplies, on the junction of Shoreham Street and Charlotte Road, opposite the Sheffield United Football Ground. That's where I get my batteries from for my chair. They are very competitive, and can supply the batteries for about 1/2 to 1/3 cheaper than going through mobility companies.
They may even help you fit the batteries on the chair if you ask them nicely.
I agree with franko's point, that the batteries may almost certainly be at the end of their useful life, if they are over 18 months / 2 years of age.
Unfortunately, you'll be lucky to get much more than about 2 years decent use out of batteries, even with careful charging, ensuring you comply with the manufacturer's recommendations for charging (such as charging them after use, and ensuring that they are put through the full eight-hour cycle to charge them fully, etc.)
A local mobility firm wanted to charge me £260 each for the batteries for my previous chair, (£520 in total) SYBC supplied them for less than £260 for BOTH! (they were huge 70/ amp-hour batteries, incidentally)
"I can't afford to live" - Stephanie Bottrill, May 2013, victim of the Benefits "Reforms" R-I-P