All you need to change the battery yourself is a long reach spanner/socket set. I think the bolt is 14mm IIRC.
10 minute job if you have the long reach spanner/socket. I invested £30 in an imperial and metric long reach socket set set from B&Q, probably cheaper than the labour on getting a garage to do one battery.
You don't use the set very often, but without it, jobs like this are virtually impossible or take 5 time as long.