Went to my tyre guy and tried to act dumb but he knew what the problem was straight away...impact damage. Price he gave me for one new 235/50/18 Dunlop SP Sport 2000 tyre was $485. Also got qoutes from 3 other tyre places but they were even more expensive...$499, $505 and $510. The tyre and size is standard equipment on BMW 7 series and Jaguar XJR so i tried my local BMW and Jaguar dealerships to see what price they could give...both wanted $550 for one tyre!
I then decided to call some used tyre shops around my area. One guy said he had some 235/50/18s in stock so i went down to see them. Picked up two used Michelin Pilot SX tyres for $100 each...both have 90% tread remaining. Also picked up a Goodyear 235/60/16 tyre to run as my spare for $30...this one has 50% tread left. All prices included fitting and balancing. So fronts are Michelins, rears are Dunlops and spare is Goodyear...i'm a tyre brand whore