The local scooter shop charged the battery for me and charged 25 quid including getting to it an putting everything back in place.
Not a bad price and a lot less than my old Honda dealer would have charged me for anything.
A few quid more would have bought you a brand new battery here.
You should probably find out how the battery drained in the first place, it should be able to
hold it's charge for years.
Make sure you close the under-seat storage properly, the seat clicks in TWO PLACES, and the
"MET IN" indication on the dash-board will light up if you don't close it properly.
It's a bit finicky, you have to press hard on both sides of the passenger seat.