Don't know personally, but I would have thought HeliAir (
www.heliair.com) or Sloane (
www.sloanehelicopters.com) should be able to advise.
Failing that, try contacting Q at HQ Aviation (
www.hqaviationltd.com) - he's flown round the world in a 44, so must have some knowledge of fitting extra fuel tanks!