[Dave Canon]

A while since I shot bikes; I used to go to Oulton Park. Your 150mm will do but you need to get quite close on a bend. You should be able to get on the inside of some bends. The pictures look more dramatic on bends and the bikes are going a little slower. I would recommend manual focus by pre-focussing on the ground at the point you will take the bikes. Also you might just as well use manual exposure if the light is not rapidly changing. You should be able to find the setting by metering the grass. Best of Luck!