When replaced are the new TPMS units self learning or must I get them coded.

thanks

From the latest manual (your car could well be different, check the owner's manual if you have one, or download a copy) :

Changing Road Wheels and Tires

If you change the road wheels and tires on your vehicle, for example fitting alternative wheels and winter tires, and the wheels have tire pressure monitoring system sensors, do the following :

Inflate the tires to the correct tire pressure.
Park your vehicle for a minimum of 20 minutes.
Reset the tire pressure monitoring system.
The system recognizes the new sensors the next time you drive your vehicle for approximately 15 minutes at a vehicle speed of 25 mph (40 km/h) or higher.

Resetting the TPMS

Only reset the system after inflating the tires to the correct tire pressure.
To reset the tire pressure monitoring system, use the information display controls on the steering wheel to select the following:

Select Settings / Press the menu button
Select Information / Press the OK button
Select Tire Pressure / Press the OK button
Select Reset / Press and hold the OK button until the system reset confirmation appears
 

Thank you I’ll check the manual 

Just to confirm after having a new sensor fitted (I bought a set of 4 from eBay only needed one)

I followed waggys guide -worked a treat thanks

I spoke too soon

my local garage popped a valve I bought from eBay into a wheel where the valve was physically broken

we then kept the other 3 originals and just put new stems on them

my mechanic then went through the learning process and his snap on detected all vales bar the new one - just did not communicate with it

i then came home (cross) and followed your guide and it seemed to work but this morning I went out again and the message reappeared- I checked all the pressures with a gauge and none had dropped so I know I have no leaks

any ideas - duff new unit? Will forscan tell me anything- like is it is not being detected ?
thanks

Can you show me link to the sensors you bought off eBay? 

There are 2 types of sensors that look identical if you bought genuine ford ones even from eBay. 

heres a pic of the one that was installed

thanks