Wiper motors being a pain to attach wheels to? Batteries not lasting very long? Sounds just like my Rex's Robot Challenge experience all over again

I'm assuming you're currently using 12V SLAs but what Ah rating are they? I found something around the 7Ah mark gave a decent run-time, and a quick search threw up these:

http://www.screwfix.com/p/sealed-lead-a ... 0x%20151mm

No idea what they're like compared to other brands but I doubt there'll be much of a difference. If you need something that'll last longer still, just type 12V SLA and the desired Ah value into Google; there seems to be a pretty decent selection for all Ah values.