Sorry to hear about the problem Paul.
Checking is pretty easy. Once the battery is re-charged, start the bike and measure the voltage across the battery terminals. Should be 14.4v or there abouts, at 2-3000 revs.
The OE ones aren't cheap, about £140 ish.
There are pattern ones about for about £60-£70. They need a bit of a wiring job though.
You know where I am if you need a hand
