Dumdidedumdide etc etc.
Listen to Devlin... or your LBS will have ten of every nut and bolt you're needing just laying on the floor around their repair stand.
Naz, mate.... a 15mm nut is designed for a 15mm bolt, not a 10. I think you're measuring across the face of the nut whereas they are expressed as "inside thread measurement" such as 10x1 which means a 10mm thread DIAMETER by a 1mm thread PITCH.
Also, if you're replacing any nuts and bolts on a bike I'd suggest NOT using Hi-Tensile because they will rust! Stainless nuts/bolts are marginally better than steel if you need to spend the extra dollars .... but lets face it, most bolts/nuts are going INTO ALUMINIUM!!!! which will strip out waaaaaay before you ever get to the point where you are using the very reason for having Hi-Tensile stuff in the first place (which is so you can do them up tighter).
There... all clear now?