Quoting my own post, I have already assembled a toolkit based on the Venhill one. It's all contained in a Wunderlich "Boxer" bag that fits beneath the rear carrier and although it's not secure as such, it's discrete enough that I'm yet to have anything disappear.
I added a few extra essentials for a recent trip to Spain and it now contains ....
1 x dayglo emergency vest
2 x breathalysers
Puncture repair kit
Torx wrench selection (T25, T27, T30, T40, T45)
Allen key selection (3,4,5,6mm)
Leatherman multitool
Combination spanner selection (8mm, 10mm, 13mm)
Headtorch
Spare bulbs
Disposable pvc work gloves
Aerosol foam hand cleaner
Cable ties
Under the seat I keep ...
Tyre valve angle adapter
Digital tyre pressure gauge
Oil filler key
1 spare headlight bulb
The underseat area is easier to access quickly so it has the few items I'm more "likely" to need, but with my Kenwood radio permanently fitted space is a bit of a premium!
Stu.