No need to remove the tank. Clean out the space around the plug as dirt tends to collect in there and could fall inside the cylinder. Compressed air is best, but a paint brush and small vacuum cleaner nozzle helps.
Obtain an 18 mm thin wall (very important) tube or box wrench. This will seem too long but there's room under the tank just above the plugs to insert the wrench upwards and then lower it above the plug.
Use a bar or screwdriver as a tommy bar to undo plug.
On replacement apply a couple of drops of clean engine oil or even better, some anti-seize compound or copper grease to the threads to ease removal next time and prevent seizure.
Halfords do a 20g tube for £3.49: http://www.halfords.com/webapp/wcs/s...egoryId_165705
Although the plug manufacturer (NGK) gives a torque setting they realise that in a lot of vehicles you can't get a torque wrench in there.
They advise that in the case of our 12mm-thread plugs, that you bottom them by hand and then turn the wrench a further 1/12th of a turn (about 30 degrees of rotation).
That's in the case of used plugs. For new ones with uncrushed sealing washer they advise rotation by 1/2 turn (180 degrees).