Simon. Not sure which part of the country that you are travelling from.
If you are in the Caravan Club you try Commons Wood at Welwym Garden city. Access is easy from the A1 and M25. There is a staion at Hatfield where you can get a train direct to London (approx 30 mins) and the site is open all year.
Alternatively Roydon Mill which is near Harlow is a nice siter and is not to far from the M11 , M25 and A10. There is also a station with trains direct to Liverpool St in London which is a journey of approx 40 mins.