V6 grand vitara, could noisey tappets be caused by faulty hydrolic lifters? If so how difficult would it be to change them?

Unlikely. The hydraulic lifters used in this model have been used across the Suzuki range for many years and have proven very reliable. The most likely cause for the noise is lack of oil pressure to the VLAs (valve lash adjusters). This can be due to the incorrect viscosity oil being used, or lack of servicing leading to clogging of the oil passages in the cylinder head or in the VLAs themselves with sludgy oil residue. There are a number of snake oils which can supposedly help soften and flush this residue out, and it would be worth trying this and changing the oil before deciding to change the VLAs - which are very expensive to purchase (24 per engine!), and are a very difficult operation to change with the engine in the car.