Hey Tom,
If the tank is leaking then you will need it sealed.
You can have a look inside by removing the fuel level sender which is accessed from the boot area and having a look through the exposed hole.
No sparks or naked flames in this area.
It is advisable to replace the sender seal at this time.
Cleaning it might be necessary.
The filter in the bottom of the tank can be removed once all the fuel has been drained.
The hex opening accommodates a hex spark plug spanner.
The Fuel Tank Tour is very informative
http://www.sl113.org/forums/index.php?topic=1712cheers