FYI,

The "sender" in my 18 gallon tank had gone faulty, the wound resistance wire had broken. 
I tried a (very) temporary soldering fix (the solder, even with a good flux, would not "take" properly to the wire). 

What to do ? Senders need to be matched to the gauge for accurate results.

Then, Karrier sent me the link to a gauge/sender supplier:
Demonmotorsportltd   http://www.ebay.co.uk/itm/UNIVERSAL-FUEL-LEVEL-SENDER-And-BLACK-FUEL-GAUGE-KIT-P-N-14508-/330729883276?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item4d0109868c#ht_4323wt_1093     A gauge & sender combo for £45.

 The 52mm gauge looks similar to "smiths" & fits just fine. It has an extra connection on the back:  12v, S(ender) & G(round/earth). Don't forget to add the earth wire (or it wont work !).   (Your little voltage stabilizing device for the fuel & temp guages will just hang on the back of it. Add an earth to it & make sure that it is mounted the right way up (see the writing on it) !).

The Sender is a bit more effort as its circular top plate is too large a diameter & does not have a fuel pipe running through it, so:

1) Empty your fuel Tank (hosepipe is good).  Undo the four bracket bolts & drop the tank.  Remove the old sender unit (using something like a large flat file or tyre lever on the lugs)

2) CAREFULLY prepare your old sender by stripping off all the old sender mechanism. DO NOT DAMAGE THE FUEL PICK UP PIPE. You can leave the remains of the rheostat backplate.  Cut the "sender wire" half way along.

3) CAREFULLY separate the circular top plate of the NEW sender unit from its "C-shaped bracket" below (With a screwdriver, It's one spot weld).  Throw the NEW top plate away.   Cut its "sender wire" half way along.
 
4) Clean the old top plate & position the new sender unit's "C-shaped bracket" against the old top plate & resting against the "fuel pick up pipe".

5) CAREFULLY attach the "C-shaped bracket" with three or four "blobs" of mig weld to the old top plate & "fuel pick up pipe".
ONLY WELD THE C-SHAPED BRACKET. DO NOT USE TOO MUCH HEAT.  DO NOT PUNCTURE THE FUEL PIPE (or you will have to use a good sealer)  DO NOT WELD ANYWHERE ON THE RHEOSTAT CASING.

6) Solder the two parts of the "sender wires" back together & attach the float (bend/cut the float wire so that it is level with the bottom of the pick up tube (NOT BELOW IT) .   Voila ! A fully functioning & matched sender unit.

7) Make sure that your "under vehicle" connections are CLEAN, connect everything & do the opposite to 1) to put the tank back. (You probably will not need to bleed the system).

Half a days work
THE END.    

Word to the wise:-  When you put some fuel back in (say 4 or 5 gallons), take a look & remember the fuel guage reading. That way you will always have a good idea of "how many miles" are left in the tank when the gauge is starting to read very low.

Thanks again to Karrier for the Link.