Mark,
Buy a replacement filter, there is a new O-ring in the box which you should fit to the housing. Also place a copper washer under the bolt head before refitting.
http://www.ekmpowershop5.com/ekmps/shops/greenmachines/cvrtcvrw-oil-filter-1383-p.asp
You may think £25 for a filter is expensive, but the minimum order from the manufacturers is to buy 10 at £30 each, so it's actually pretty a good deal!
Chris