Technically both of you Garry and GeorgeB are correct.
I too use a standard 39cm dish and even on my hols to the south coast last year recieved a perfect signal, although it was a little trickier to line up, i always achieve a perfect signal when ever i put the tripod down, the pegs through the feet ensure it stays solidly fixed even in high winds.
The signal does get weaker down towards the south coast and if you cross over into europe you will certainly need a bigger dish.
I would agree that sporadic signal is usually down to natural causes, such as trees or weather, or even an electrical interference of some sort from somewhere, maybe an electrical pylon?
I dont think that a dish can be blamed entirely for bad signal, there are other factors to consider, such as the quality of cable, sky recommend a cable called ct100, its a good copper shielded co ax that is very resiliant to weather conditions. perhaps georgerb has a slight break in cable or a standard tv coax, i have even seen boxes being the fault for low signal, a badly soldered joint inside the box for example could cause all manner of faults. if you believe the dish and cable is ok and nothing is blocking the signal i would be looking for a replacement box.
your average sky engineer is an idiot so dont think that you are calling the experts when you call them, you are most likely getting a guy that before his 2 weeks of training was skinning rabbits for a pie manufacturer.
As garry stated, a different engineer may be a good idea.
in summary, a 39cm dish should do most of mid uk and down to the south coast comfortably but greater care needs to be taken to get that perfect signal.