I have a 67 plate fiesta from new 13000 miles would this still be under warranty as I am having problems with a rear wheel tyre pressure kwik fit changed the valve at the weekend and again already losing pressure so annoying any help with this would be greatly appreciated thanks 

probably depends on the cause. eg if due to a manufacturing defect in the wheel then probably yes. but more likely it is a tyre problem.

has the wheel been taken off car and put in tank of soapy water to see where the bubbles come from?

but even that does not always establish the leak as the leak might only occur when there is pressure (weight of car) on a certain bit of the tyre.

A friend had a leak which the garage failed to identify a couple of times, but eventually it was found to be a staple in the side wall. it looked like someone had fired a staple gun in the side wall and the staple was embedded so very hard to spot and it only seemed to leak air when the weight was flexing the sidewall where the staple was.   Of course that could not come under warranty.

6 hours ago, Gary180 said:

I have a 67 plate fiesta from new 13000 miles would this still be under warranty as I am having problems with a rear wheel tyre pressure kwik fit changed the valve at the weekend and again already losing pressure so annoying any help with this would be greatly appreciated thanks 

Ford won't help under warrenty if youv'e had work done by Kwik Fit - (and I doubt tyres would be covered anyway).
If you took it into Kwik Fit and asked them to fix the pressure probelm and its still happening the you need to go back to them, or take it to another tyre place and ask them to have a look,

Other option is to buy a can of the tyre leak foam and squirt some of that in! (that will make it difficult to repair if you have a puncture though)