I have a new belt, a 14mm socket and a breaker bar. Do I need:
I won’t do it until the car has had its first run of the year, as there is a bit of surface rust on the pulleys which needs polishing off. It has been an unbelievably damp winter.
Removing the battery and surround is a very quick job and does make the whole thing simpler, the surround does come off by itself so you don’t have to unbolt the tray that supports that battery. I can’t remember how it is attached to the tray right now though
Easy - See Below
Just don’t drop the battery clamp retainers into the undertray when you have removed the retaing nuts as you will have a dreadful job to get them back!
Get the correct belt, air con, non air con types. See diagram below how it goes, release the belt with the tensioner pulley. The aircon models will have the larger pulley bottom right 
Also be prepared to check the tensioner and idler pulley(s) for grumble/click.
They should be silky-smooth and silent.
At (cough) 17,000 miles, they’d better be!
I did that years ago when changing the battery. Luckily I have stacks of M6 nuts and washers kicking around, so I didn’t bother looking for the lost nut.
Done. Worst bit is unclipping the battery leads from the battery box. I unclipped the top part of the battery box but just moved the base out of the way.
It’s quite easy to get the belt sitting on the edge of a pulley but I had a final check and also turned the engine a bit using the crank pulley.
Then rolled the front wheels onto a few pages of glossy magazine to turn the steering lock to lock and reset the DSC.
