Stratton is in a dip which doesn't help. Unfortunately, Cornwall suffers a lot with poor mobile coverage - especially along the coast of North Cornwall. However, I have always found Orange to be the best, often being the only service with any signal at all. If you are already with Orange and still can't get a signal, then there's no hope!!
Well I use 3 for my mobile broadband and T mobile for my mobile phone work and both seem to to work very well. That was for the Wooda Farm caravan park in Poughill and now for where I live in the Flexbury area. I expect the service qualkity to drop during the month through to early September as there will be a lot of visitors using the services
I did quite a bit of research into this. You can find the closest base stations on this site: http://tinyurl.com/4scf7z
Briefly - there is an O2 one at Kings Hill. There is one behind the bowling alley building that covers Orange, Vodafone, 3 and T-Mobile. There is another Vodafone one north of Bude past Bush near Ivyleaf Hill. Finally there is another set covering the other carriers further north on the edge of the golf course.
I tried SIMs from Vodafone, O2, Three, Orange and T-Mobile and found Vodafone to be the only usable one in Poughill due to the proximity of the cell near Ivyleaf Hill. The best thing you can do is order free pay as you go SIMs for every network and try them before you sign up for a new contract.
The other solution is to buy a repeater. If you search for GSM repeater you will find them on ebay or Google it. GSM900 (O2, Vodafone) repeaters are about £90. GSM1800 repeaters (Orange) are about £180. I installed a dual band one GSM900/1800 that covers them all which cost £270. Now I get full bars on Orange and O2 when I used to get no signal indoors.