When travelling to the USA via the 'Visa Waiver Programme' (from England) how many days can you stay ?

... I am aware that you can stay 90 days or less in the USA when travelling in from England without having to have a visa but does this mean 90 days in the whole year or 90 days in a single visit ? I want to know because I want to spend one week per month in California and I am wondering if I would need to get a visa in order to do this, as that would add up to 84 days per year (less than 90). Hope this makes sense !

90 days in a year I believe.

90 days at one time...then you must return home and come back if you need to....leaving and going to Mexico or Canada does not count.

90 at a time. You can re-enter the USA again on the VWP in the same year. The officials at your POE may get suspicious but a good rule is to spend more time out of the USA then inside ... for example if you go for 90 days don't then come home for a week and go for another 90 days.

That said it depends on your luck on the day you arrive. You want to prove that you are only on holiday or business not intending to stay in the country or are doing anything illegal.

http://www.visa-requirements.com/ try looking on this site

There is no legal limit how often you may enter the United States under the visa waiver program. You just need to convince the immigration officer that the purpose for your visit is tourism. I have a friend who is a retired German Army officer. He has spent 270 days in the USA last year. When entering he shows a 50 pages photo album which has one page for each state and about 25 empty pages of the states he has not visited yet.
If a younger person however will ask for frequent entry the common sense would ask how is somebody able to finance monthly trips to the USA? Either by illegal working or criminal energy and the chances of getting entry denial become higher.