Fuel Gauge Problem

I've looked through various threads on fuel gauge problems and I'm confused. I've read through the Haynes manual and it's not been any much help either...
The fuel gauge isn't working on my 2002 SWB Defender Station Wagon (not a conversion). Normally in these situations I'd go to the sender and earth out the wires and check the gauge etc but the gauge sometimes registers a quarter and once even half but the fuel low light comes on and off from time to time as well.

I only picked the Defender up yesterday so I'm not that familiar with it! Do I have to drop the fuel tank to get access to the sender unit? I'm reluctant to order a new sender as I've read of people replacing them and not solving their fuel gauge problems so I'd better have a look first. Any hints and tips please?

The sender is likely the problem or its connections.

Lift the floor panel for access.
Check the earth wire or earth the tank and see if it works

There's some heavy rubber fitted under the rear seats that will have to be unbolted. I'm happy to do that but I thought that on this model there was no access from above and you have to drop the tank? Just don't fancy unbolting three seats etc to find no access hatch.

Its a tank off job to access the fuel pump which incorporates the fuel gauge sender unit, not a cheap unit to replace, I would not recommend the cheaper pumps on the market, go for a German manufactured pump.

I've just been quoted £600 to fix the problem so it looks as though it will be me on my back with the Defender on ramps!