Clive (Cardiff),
I have had a 3 dongle on monthly contract for a couple of years.
I have found that coverage maps are only a guide, irrespective of which companies dongle you use. I have been on sites with friends useing other companies dongles,reception sometimes varies between the dongles.
Also on some sites the strength of the signal, or whether you get one at all can depend on which pitch you are on.
For example at Malvern c.C. site i can get a signal strong enough to aesily send photos via e-mail, yet on other parts of the site i cannot get a signal.
One good thing about a dongle is you can type your e-mails in the caravan, or sat outside in the sun having a bear, then when you go to say the supermarket or anywhere with a signal while the wife is shopping plug in and send your e-mails.
Hamer