V8 Head gasket

Exactly a year ago (12,000 miles) I had my head gaskets/seals replaced (both heads) on my V8 Mk 2 Discovery. Today I've noticed what appears to be a leak again (by there the VIN number is etched on). It went 150,000 before the first work; should it be that it only goes 12,000 miles on the new gaskets? Can anybody help? Thanks

do you run on gas?
are you using fully synthetic oil?
is being oil changed twice yearly?
were they good quality parts?
it should still be under warranty?
is it the rocker gaskets with oil dropping down?

If the gaskets used on the rebuild were steel and not composite, then they tend to fail early, were the heads checked/skimmed at the time?

Peter