Items to Carry

I've divided these into essential and aspirational

Essential are those needed if a bike shop is readily available. Aspirational are those items that may be needed should you be touring with no easy access to any spares. Naturelly there is interpretation, these lists are a guide.

  Comment Illustration Will this STOP you? Likely (in my opinion)
Wheels        
Tyre Levers You'll need a minimum of three   YES 5-Probable
Glue and Patches All sorts of varieties available   YES 5-Probable
Inner Tubes Of the appropriate size   YES 5-Probable
Spokes

Suitable for your bike. A great place to keep them is to tape them to your rack

  NO 2-Occasional
         
Spoke Repair An ingenious fibre repair https://www.sjscycles.co.uk/spokes/fiber-fix-emergency-replacement-spoke NO 2-Occasional
Spare Tyre Check the size of your tyre, folders are available   YES 2-Occasional
Tyre Boot (If cut the sidewall) Can use a dedicate tyre boot, a piece of old cut tyre (cut into suitable lengths with the bead removed) or a piece of thick plastic   YES 2-Occasional
Tubeless When running tubeless, spare valve core, small container of sealant for top ups. Dynaplugs (worms) for repairing puncture holes.   NO 3-Possible
Chain, sprockets and gears        
Gear Cable Make sure you can fit it   NO 2-Occasional
Chain Breaker Often present on multitools   YES 4-Likely
Spare links Just make sure you have the right widths, 9,10,11 etc   YES 3-Possible
Spare Chain If don't take links. Suggest 5 sections     1-Unlikely
Luggage        
Pannier clips They do occasionally break   NO 1-Unlikely
Spare Bolts For racks etc   NO 3-Possible
Brakes        
Spare shoes/pads If on a longer tour, very occasional fail   NO 1-unlikely
Brake cable Make sure you can fit it, especially if internally routed      
Outer Break sleeve If you have internally mounter brake/gear cables (go through frame) carry on of these. In an emergency you can zip tie to the frame and use this for the cable   NO 2-Occasional
Brake fluid If hydraulic brakes and a bleed kit   NO 1-unlikely
7mm spanner to bleed brakes If hydraulic brakes   NO 1-unlikely
Other Bike Parts        
Seatpost bolt It's happened to me, it cracked and I had to ride the saddle down   NO 1-unlikely
Chainring bolts     NO 1-unlikely
General        
Spare Cleat for SPD's They do fall out and unless you have toe clips  

NO

1-Unlikely
Bolts for Cleats They do fall out (M4 or M5?)   YES 2-Occasional
Derailler Hanger Hundreds of different types, very difficult to obtain
One that claims to fit all and useful in emergencies https://www.weirdosonbikes.com/product/emergency-mech-hanger/
 
YES 1-Unlikely
Zip Ties Always useful   NO 3-Possible
Electrical tape/duct tape Always useful   NO 4-Likely
Specific Tools        
Freewheel Remover If you want to get at spokes if broken, lots of different types   NO 2-Occasional
Pliers These are superb for removing fine pieces of wire and they are small
https://www.knipex.com/en-uk
https://www.knipex.com/en-uk/products/pipe-wrenches-and-water-pump-pliers/knipex-cobra-high-tech-water-pump-pliers/knipex-cobra-high-tech-water-pump-pliers/8701125

  2-Occasional
Multitool So many uses      
Allen Keys The large one to tighten the cranks are very useful      
Others        
Toe Straps So many uses and small      
Voile Staps Useful https://www.voile.com/voile-straps.html