Just why is it they think it was arson?
Signs of accelerant?
Evidence of suspects seen toting a Flimsy of Petrol and smoking a fine Cuban Cigar?
Someone drop a cigarette lighter with his name on it?
Makes one wonder why your ordinary street thug or second story man would would want to poke the Plod in the eye like this?
DC electrics being what they are....shy of some solid evidence of arson I would be looking at an electrical fire that got away without being observed until it had gotten well advanced first then start on the arson avenue.
Reckon the price of a bit of fencing, barbed wire, and a few motion sensors combined with one of the Big Brother TV cameras the country is covered with would have saved a bundle of money. For that matter, a Plod could have been stationed there all night for years for what this will cost.