Ensure you have breakdown cover either in insurance or buy it separately.
Depending on space:
A mars bar.
Mobile (make sure the damn thing has charge) - this should become habit).
V sponge - look it up
Scarf / neck tube spare
Torch (decent LED one with batteries with a long shelf life or wind up one)
hivi (breaking down in the dark may = no lights).
spare cash - a note in a waterproof thing is good.
Plastic bags (leaking boots, pockets, etc all good)
Condom - loads of uses for these things - ever had a glove finger leak?
All in a drysack or in waterproof pockets.
multitool (a good one - see ride magazine product tests).
WD40 / 3 in 1
duct tape, plumbers tape
tools such as carefully selected spanner sizes, etc.
puncture repair kit (ideally just ultra seal / puncture safe your tyres - look it up).
Hand warmers - either those little packs or a tin with some ethanol based fuel and a lighter.
tampon - an oil / fluid leak onto a tyre, etc can be made safe for the ride home with one of these. Also good for starting fires if you end up in the arsehole of nowhere waiting in the freezing cold for the breakdown truck.
People will say first aid kit but really, they won't save your life or do much more than patch a boo boo. If you're going to carry one, then carry a proper compression bandage and a tourniquet in a pocket as severe bleeding is the only thing you're likely to be conscious enough to fix and can't wait.
I'm sure I've missed things.
One thing I will definitely suggest you do is get a better bulb - check the fitting (likely H4) and swap the bulb for a Philip's extreme vision bulb and nothing else. The difference to how well you can see is amazing. If you can, also swap the horn for a Stebel magnum - it's a straight swap, quite cheap and can save your life as the small ones that they fit just can't be heard.