KTech charge around £90 labour for the shock service, the spring is about another £80. Plus vat. On the plus side, you do know the spring is right for yer weight.
I had my OE shock serviced and resprung in 2008 , then bought the full monty shock upgrade in 2009 ( I bought a second cheap shock off ebay as a donor). The full monty is better , but I was pleased with the upgrade of the OE shock, and still have it as a spare.