after five years it owes you nothing. Definitely time for a nice new one !
£120 sounds waaay too much for me, although Bosch not the cheapest option of course. (IMO Halfords get battery sales based purely on their high st presence/accessibility rather than competitive prices...)
Got a new one for my 9-5 for £62.04 from these people a couple of years ago
http://www.shieldbatteries.co.uk/ Lots of good battery info on website broken down by car model, although looks like you have to call 'em for price
Maybe not ideally located for you (but at least Herts so same county...!) Otherwise well worth checking around for online suppliers, many offer next day delivery. Mine came with 2 yr g'tee but no issues at all so far
Don't recall need for any special procedures re alarm etc when changing old to new, maybe others can confirm