The perfect OSRAM spotlight just turned-up in my local DIY shop - so I have had a go. I am very pleased with the results. I recon I have made a bike light for around £20 that would retail for well over £200 from a bike shop.
Step 1: Buy The Bulb
This is a 240Vac 4.5 Watt replacement for 35 Watt GU10 halogen spotlights. It has a narrow 25 degree beam angle.
There may be better units to start with to be found if you scour the internet, for example 12V, or single LED rather than 3 LED array. I liked this OSRAM one because it is near watertight, comes apart with screws, and looks the part. (Unfortunately I didn't take any photos of the unmodifed spotlight - but you can imagine the GU10 terminals in the place of the switch and lead exit in the photo below, other wise it is unchanged externally.