Cambridge To London
Travelling from Cambridge to London could not be easier by train. There are regular trains, at least one an hour, which travel nonstop, and many other slightly slower ones during each hour. The last train back is at around midnight. There is no more sensible way to travel. The trains arrive at Kings Cross. There are others which arrive at Liverpool Street, but they take longer, and Liverpool Street is usually less useful for the centre of the city. The journey to Kings Cross takes only an hour, at the weekend a return with the tube (aka One Day Travel Card) will cost you �19.10 on the day, or �12.70 if you have a discount card.
Do not, though, be fooled by timetables. Whilst the trains are regular, they are often delayed and the timetable shot through. If you turn up at the station, within two hours you will be on the tube, at the outmost. Refering to a timetable and arriving for a particular train can only cause headaches.
National Express also have a cheap and frequent service from the centre of Cambridge to London and back. About the same price as the train and takes only a few minutes longer. Usefully (for some of us) the coach calls at [Wanstead]? and Stratford station.