Ford Focus won't start in cold?

My Focus 1.8 Zetec 1999 has a weird starting problem when the outside temp drops below 5 or 6 celc.
When trying to start nothing happens, won't turn over or anything. What does happen is the immobiliser light on the dash flashes quickly for approx 1min then flashes a code. this code is 15 which apparently means " wrong transponder Key"?
this happens on both keys!
Eventually after about 30 mins of trying it will start and be ok for the rest of the day.
if its a bit warmer it is fine, no problems.
Have tried lots of things from ignition barrel to ecu and Ford are no help at all. Can anyone help please