Hi Ive got a 54 plate Peugeot 307 which has got an oil leak. ?

Oil can leak from a faulty head gasket as there are a number of oil ways from the crankcase to the head. These feed the valve gear. I had exactly the same issue on my diesel 306. Yes £600 is not far off right. But there is nothing to stop you phoning around for estimates. Try Halfords Auto Centre if there's one near you. I have found them to be surprisingly honest.

WAS this engine EVER over heated?? i always replace the COOLING FAN SWITCH on cars at 100,000 miles or LESS as INSURANCE! You MAY get away with repacking ONLY the HEAD BOLTS that cos about $30 a set in AMERICA! Get the engien HOT as you can, then REMOVE the OLD BOLTS as you install NEW ONES! OR just realize you MUST remover the cylinder heads And do things the RIGHT WAY! You will need NEW BOLTS anyways for this job! r car should  get to 500,000 miles EASILY! 

Find another mechanic to work on your car. This one is ripping you off.

When it was registered doesn't matter to my knowledge unless your area of Europe also has some kind of wackey law on oil changes depending upon when a vehicle was registered.

If the head gasket is leaking you will have either oil in the coolant or coolant in the oil, a pretty noticable loss of power, and (yes) an oil leak that's pretty easy to find where the leak is coming from. If you don't have those symptoms my guess is it's a valve cover gasket which is a much easier and cheaper fix.

Head gasket, I doubt it. Valve cover gasket very possible. A valve cover gasket should cost under $200 to repair.