Hi
my 06 had its spark plugs changed in its 4th year service which was a big service which according to the info I found on volkswagens website under the owners section is correct. Just had my 5th year service which was a minor service for £129 under the fixed priced servicing scheme, also says in year 5 you should have your brake fluid changed and air con refreshed.
Year 6 is a major service including plugs, and air con service. Check your receipt see if they did plugs in your 4th year if they did'nt, ask them why not.