The boys oset 12.5 24v was charging while I was working on my bike, it's a std oset running yuasa batteries which charges by pluggjng into the bike.

Anyway, I was wearing shorts and my leg brushed really, really lightly on the footrest and I could feel the electrical current. When I tried it with the palm of my hand I couldn't feel anything, it could be felt using the back of my hand.

Is this normal? If not, what may be causing the problem

I personally think it may be just static or something that feels like passing current. You  would have to have a wiring fault on the bike plus a charger fault plus the right conditions for you to pass the current for it to be a problem. If you want piece of mind then get a volt meter and measure AC and DC volts from the footrest down to ground.  

ok, I'll give it a check

I asked my technical guru and his opinion is that it is a badly isolated transformer in the battery charger, suggest you try a different charger.

He suggests using a neon bulb, mains checking screwdriver to prove the point, just touching the bike will light it up proving the theory that mains is present.

We do not think it is static this usually just gives a crack on discharge rather than a tingling that you are probably feeling.

yes a badly isolated transformer will cause these symptoms  only on a very primitive charger but as far as I know the oset chargers are switch mode so therefore should be isolated anyway.. but the whole bikes wiring should be isolated there is no common on the frame so it will also have to have a wiring fault .. its true and unlikely to be static.. as nearly all times it is a single crack, but not always .    

Isolated transformer? Single crack? Is that something to do with the oil filter!!!! Pardon my ignorance, but I'm not up to speed on these new fangled digital bikes. Can you please enlighten me?

:-)   not really anything to do with the bike, more to do with electrics and mains power supply ( and maybe static ) , can get the same sort of problems with anything that is plugged into a the mains but it is rare.