I did this perm with a mate in November,a really nice ride. We parked in the park and ride at Trumpington, it's about £1 for the day. There's then a great traffic free cyclepath straight into the centre of Cambridge that takes about 15 mins. i've also done Nicks Cambridge market 200, another great day out - recommended. Nicks perm routes are great for London based riders like myself with a fast simple journey to Cambridge by train or car and then you cycle straight into really nice countryside.