So I made the foolish mistake of leaving my already old and decrepit battery pack in the shed over winter and a few very cold nights appear to have done for its ability to hold any charge. I decided that it was time to replace the battery pack and get myself an external charger so the little thing doesn't need to sit in the kitchen to feed. (I have no power outside the house)
A quick look at the uk distributor suggested that perhaps UKP140 for the battery pack and UKP99 for the quick charger was a mite expensive. Time for a little bit of DIY I think. A bit of applied pressure later and I had the top off the battery pack.
It turns out my pack was made up of two Yacel Y17-12 lead gel batteries. These are no longer made but the nice chaps at Yuasa suggested the replacement model NP17-12. In the end I went with NPC17-12 which is designed for cyclic applications such as the lawn mowers and golf buggies. (Notice the extra C) In the end I picked up a pair from BatteryMasters for UKP 68. You need two 12v cells rather then one fat 24v otherwise they wont fit in the packaging unless you are willing to do some more serious modifications to the holder. Depending on how your battery pack in configured you might also need to pick up some crimps as I need to convert from spades to bolts.
Finally while I was at the shop I picked up a charger; but in retrospect I should have gone with the 4amp version which is now 1 UKP less. Still it charges the entire battery overnight which is much better that the standard 22 hours charge time for the standard power adapter. You don't need any special connector, just use the supplied bulldog clips on the big bits of brass on the back. The red one should go to the slider nearest to the fuse. If you get it wrong the LEDs on the charger should flash. No more mower in the kitchen.
I would think that doing the battery upgrade should be doable by anybody who can wire a plug or do simple car maintenance. Just make sure you keep track of polarities when you take the old batteries out and put in the new. As with anything you read on the web though you do this at your own risk etc. etc. results might vary, you might well cause your robomow to burst into flames.
140UKP saving for a little bit of web searching, well worth it. Battery now lasts far longer than it takes to mow the lawn which is better than when I bought the thing refurbished to begin with. Oh and don't leave your battery out in the frosts!