i'm with purf on this, 95% chance if not more it'll be tank seams...
Its a diy job if you don't mind fondling the spanners - my only words of warning before starting a tank swap is:
1. Don't do it when there is lots of fuel in it as clearly its gonn abe heavy and you will spill it everywhere
2. Liberally spray the bolts that hold the tank on with penetrating fluid - i would use something actually designed as a proper penetrating fluid like PlusGas over WD40 for this. Spray them and leave it and do it again the next day for a few days to it properly works in - and when you go to drop the tank don't just yank on them really hard. Turn it a bit and then go back the other way and keep doing that, spraying more penetrating fluid as you go, you really don't want to snap these bolts and they have been under the car its whole life.
3. When the tank comes off there is a breather pipe, the bit where it goes on the tank has a little rubber sleeve - you need to retain this and put it on the new tank and its always sticks to the old tank and doesn't come off with the pipe, you put it back together without it and fuel will leak like mad when its over half tank. Ask me how i know
Other than that its really simple, just undo a few bolts and pull of a few pipes and bang new one in in reverse. Remember to disconnect the fuel lines and electrical connectors from the top of the sender unit (under the rear bench) before you start lowering the tank also.