Well, I hate to admit failure.
But unfortunately it transpires that I’m not a mobile phone antenna boffin. Because of the extremely poor reception where we live, I constructed a Yagi antenna, with the directors and dipole cut to the correct length and spacing for a frequency of 900MHz (GSM). I used a length of PVC pipe, and connected some 75Ω coax to the dipole via a balun. Here’s the constructed Yagi: (excuse the mess on the workbench!)
I then leaned the ladder against the end wall of the house, and balancing precariously on the top, connected the antenna to the gable end, using another length of pvc pipe for the mast. I pointed the Yagi at the closest mobile mast (apparently 2km away, due East). The coax cable was fed through a gap in the brickwork just below the roof tiles, into the barn.
The other end of the coax was stripped, and the copper braid bent back, to form another dipole suited to the 900mHz wavelength (each piece of copper being 83.25mm long in case you were wondering; these two pieces in total equate to half a wavelength). I stapled this coax to a beam in the barn, and came back inside to switch on my new Blackberry.
Before installing the aerial: Zero signal.
After installing the aerial: Zero signal.
So at the moment I’m a very disappointed puppy.