I've run into the same problems with reception as those above so emailed O2, here's their reseponse:
Thanks for emailing us and I'm sorry you're having problems with network coverage on your iPhone.
We don't have a reported fault with the coverage of iPhone. Your area shows standard coverage on our online coverage predictor. However, the map does show a prediction of coverage rather than a true measurement. The geographical and structural features aren't taken into account so the actual coverage experience can vary.
We're always expanding our network and it's only through feedback such as yours that we can find out where our customers' needs are greatest.
To check the coverage, please click the following link:
http://www.webmap.o2.co.uk/
Please call us on 08706 003 009 for help in this matter. Lines are open Monday to Friday 08:00 to 09:00pm, Saturday and Sunday 08:00am to 08:00pm. Calls from O2 Pay Monthly mobile phones are free of charge.
You'll be asked to provide us with the following information:
a) mobile number affected
b) alternative daytime contact number
c) phone make and model
d) time and date the fault occurred
Please also have the answers to the following questions:
a) Is the fault temporary or continuous?
b) How long has the fault been occurring?
c) What error message is seen?
d) What error message is heard for example, 3 rising tones?
I appreciate your patience with this matter. If there's anything else we can do for you, please reply to this email.
Please visit our online Help Centre at:
http://www.o2.co.uk/help
Important - When you email us please provide: your date of birth, postcode and mobile number as it helps us answer your query faster.