Anyone care to put a name to it, and a little background would be interesting too...

David Nash wrote in reply;
"It looks to me like a fine photograph of a robber fly - A member of the family Asilidae (within the order Diptera). I can't tell you which species I'm afraid, but the strong, hairy legs and "beard" are pretty typical ('though I'm not a Dipterist, and there are a few other small families of similar large flies. It's definitely a Dipteran!). The robber flies are all predators of other insects, often catching them on the wing, after which they take the insect to a roosting site and suck out it's innards... which
sounds exactly what you observed. They are also some of the biggest flies (i.e. Diptera) - I remember seeing some in Australia a good three inches long."

Bart Vanholder replied;

I promised the name of the predator fly you had in your pages: Last friday at the entomological meeting we had a glimp at your pic of the fly and the butterfly. It turned out to be Dasypogon diadema (Fabricius).
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Dear Mr. Coombes,

I found the photograph of an asilid fly on your web page "andal15.htm", which is named "Dasypogon diadema". I must correct this, you can find a b/w photograph of this species on:
http://www.geller-grimm.de/dasy.htm
D. diadema is a bee-hunter and a member of another subfamily (Dasypogoninae).

Your photograph shows a member of the subfamily Asilinae. I believe that this animal is a species of the genus Machimus and the species-group of setibarbus. A better determination is not possible, because you have to check the genitalia of this male. [I need also the place of this record]
Robber flies catch a lot of different insects, most of them are not specialized, an important parameter is the size of the potential prey.