I have always ride XC bikes (current Marin Wolf ridge) but now have decided to get a Kona Coiler Deluxe. My current setup on my MArin had mavic rims with 9mm axle – i have now purchased some 20mm qr forks. Is there a way i can keep my current wheels, either get them rebuilt (or is this pricey) with a new 20mm hub or try and get hold of a new front wheel?

Yes, you can get them rebuilt with a new hub but it may be easier to sell the old wheel in the classifieds and then buy a replacement there too to fit.

Rebuild including unbuilding the current wheel is likely to be £20-£30 I reckon and then add in the cost of the new 20mm hub. Less if you unbuild the wheel yourself (which is easy, just keep the spokes from each side of the wheel separate). That said your shop may refuse to build with used spokes as it's usually a bit harder and no doubt they'll claim BS such as 'you shouldn't reuse spokes'.

just get a new 20mm wheel

you can pick up s/hand Hope hoops for £70ish