Richard
Before cutting with a hacksaw, I'd try to find a larger allen key by switching between imp to metric, or even use one of those many splined torx-like things. Further use a little gas flame from a welder to warm up the bespoke bolt, and finally use penetrading oil, which unlike WD40 is heat restistant. I've got a nice product called "Biral" in a red can, which is superb for the job, but doubt it's available in England.
If you need to drill out the screw you must know that these b****rds are often very hard to get through without damaging the thread, especially if you haven't got the right tools.
/r