Short-circuit explanation?

Sounds like some prat has switched the neutral instead of the live. Hence when you turn the switch off there is still 240v at the live. When you measured with your meter you actually measured across live and a totally unconnected bit of wire!

Best thing is to turn off the leccy at the main box if doing any work on the sysem.

TonyB

What Tony said. Plus, I always use a mains tester of the type where you put your finger on the end of it and it lights up if live. And I check all the wires, just in case. I once got a poke in an old cottage where someone had done some creative wiring and the light I was working on was fed from somewhere other than the switch.

Steve

If I understand you correctly you have one twin/earth going to your fitting. Firstly when you test anything is dead you should always test to earth and not to neutral. Now to understand your problem. If the circuit is as I said above then if your wiring is correct then your fitting will be dead when switched off at the wall. Clearly it was still live so there are two possibilities. Firstly you hadn't actually switched it of at all when you chopped. Secondly your switch is on the neutral side and not the live. This would mean that when switched the voltage between your two conductors would be 0V as there would be no volt drop across the bulb due to no current flowing. There would be 220V though between both conductors and earth which is why you blew your fuse. If this second possibility is what you have then this is a very dangerous situation and should be put right ASAP. Switching the neutral is potentially fatal even by just replacing a bulb.

First mistake - turn off the power at the fuse box!

Lights are wired in a number of different ways dependant upon when, how and why!!

Wickes do a good leaflet that illustrates house wiring (Good Idea Leaflet

That said live and neutral are often taken to the ceiling rose and then on to the switch - that 'may' explain the bang.

Peter

may be a permanent live (as there is in some wiring methods)

I think the OP has only one twin/earth at the fitting

Who is to say that the original fitter used the correct cable/colours (Its frightening what you come across)