i have seen lots of new caterhams/westfields and second hand ones that dont have wing mirrors and got me thinking is it a requirement to have them or are these pictures only for advertising reasons.

You must have a rear view mirror plus a drivers door.
If no rear view, then a passenger door mirror is also required.

They will be needed to get through SVA,
They may be needed for future MOT's
They are certainly needed to keep an eye on the chav that is behind you trying to spoil your day.
Steve

My car passed with just the one wing mirror on the drivers side....though I also thought you needed at least two to pass.....Oli.

You did well to get through with just one Oli,
How big was it?
Did you borrow it from the wardrobe?
Steve.

Legal requirement for new vehicles is at least drivers door/wing and interior mirror . N/S needed if no rear view from interior mirror

You must have a driver's door mirror and an interior mirror. If your interior mirror does not give the required field of view then you also have to have a passenger side mirror