Hi Paul,
When we were there last summer, we were pitched on the field at the top of the site, and i found coverage was fine for phone calls. However, my husband remembers he had to go and stand at the top of the hill near the wardens caravans in order to get a signal!!
We're both with T mobile, and at time had the same model and make of phone, but he's always had problems where my phone has been fine, so i think the site coverage (at the top of the hill where we were anyway) was ok, i think he just had a duff phone.
We also had a relative to stay for a couple of days, i think his phone was ok too, but i'm not sure which network he was on.