500W Floodlight from lighting ring?

[Sickboy]

I want to install a 500W PIR floodlight outside my house. The only place I can put it is above an upstairs window and I want to take power from the loft. The only power the loft has is for the upstairs lighting which has its own circuit breaker in the consumer unit. That circuit currently feeds just 3 lights and an extractor fan.

Can I fit a 500W light to this lighting ring or does it need to fitted to the ring main due to the high wattage?
If I can, then how? The easiest way is to install a junction box in the loft and run a spur straight to the light. I'll put a switch in as well but does this need to be fused? The circuit breaker is labelled with B6 if this means anything.

 

[andy]

best use a FCU from a ring main. from the lighting circuit and it might cause problems

 

[plugwash]

in terms of total load it will be fine on the lighting cuircuit (assuming your other 3 lights aren't massive)

but there may be issues with switch on surges tripping the breaker