You said you paid 799 **Pounds** ?? (Forgive me, I'm an American. They don't teach me those things in this country. =\)

That's what? Say... $1000 - $1200 US Dollars? That's really not a bad price AT ALL for a pre-constructed electric bike. That said, I do agree with Chvid. If you want to do this right, take several weeks to study and read all the information you can. www.ebikes.ca is full of information and high quality hardware. This forum is great. The "V is for Voltage" site is great.

There is a wealth of general electrical information here... http://michaelbluejay.com/electricity/general.html
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If you are going to build your own e-bike, I would actually recommend you build two. Build a cheap one first. Small motor, Sealed Lead Acid batteries. This will teach you a huge wealth of information and only cost you $300 or so.

Then you can buckle down with your new found knowledge, and build your dream machine.
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I just happen to be building my dream machine right now. =P I made this video last night for Jethro, but it's bike building tips, so I figured I'd post it here too.