Iv got 250w HPS and an RVK plugged into an extension cord. the 250w, plus a desk fan are plugged into a contactor, which is plugged into the extension cord. does this sound safe?

cheers

NO ballast should be plugged into main always no exceptions better to be safe than sorry mate.

That'll be fine dude, best off stickin an RCD on the end tho..

Yep its fine.

I got my ballast plugged into my contactor which is plugged into my extension lead

yeah it's fine, i've got a 400w and a 250w and 2 fans and a dehumidifier plugged into an extension. as long as the extension is rated to 13a and you don't exceed that load then it's fine. Always make sure that the cable is fully extended and not coiled anywhere though as coiled cables build up heat and can set on fire.
(w(att)/v(olt)=a(mp) - uk supply is 240v) bear in mind that a 250w light, which will be 1.06a will pull more amps than this when first turned on as it needs a charge to hit at the start, so make sure that you leave some headroom in terms of amps. (you've no problems at the moment)
always worth putting an rcd in at the mains plug point though as suggested above.