Thanks for your comments.
A quick update...
I have now replaced the radiator and what I find is that after filling the coolant as directed, when the car is cold, making sure that all air has been removed from cooling system.
I usually drop my kids to school in the morning and the car starts to get warm, at the same time the cabin heating gets warm too.
I then drive to work, which is approximately 13 miles. I park up, usually in a residential area near work at just before 9am.
When I return to my car at about 6pm, I find a big pool of water under the car and I have to refill the coolant.
It definitely is an air lock as the pressure is building up, therefore causing the coolant to leak out from under the filler cap. But I cannot figure out what is causing this airlock.
There is a housing into which 4 of the hoses connect to, it is located near the throttle linkage, could this be my problem area, or even the heaters in the car ??
Thanks in advance.