Hello Dave,
As you say, unwarranted warning lights can be due to a below par battery, so well worth getting its efficiency checked. Halfords should be able to do that while you wait, and is usually free, so zero cost elimination - or confirmation that it is at fault.
Again as you say, it may genuinely be due to a failing ABS sensor, and you would need diagnostics to check if that is so, and which wheel is the offender. This would be option 2 at cost.
Perhaps you could let us know what the outcome is David.
Kind regards,
Gareth.
p.s. If the battery tests below par then it may be wise to let us know before committing to buy.