Any tips to look for. The sender seems to give varying resistance from 0-ish to about 100ish or something ohms. So I suspect it is in the wiring or something else.

Every now and again the guage starts reading, I thought it was just the float, but its now fixed and guage not working again.

The float moves a brass arm that rubs up and down on a copper coil, varying the resistance.
If it reads sometimes I'd say the arm needs more pressure, easy to do once you pull the sender out.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

How do you do that?

Also, would that still be the case, I mean it varied it's resistance when testing with a multimeter, and so I tried plugging it back in and doing it with someone watching the guage and still no go??

I have a majic super rare one....... It is always half-full it never runs out

How? You take the cover off it and bend the slider a bit so it rubs harder.
I agree though if it reads reliably over its range it is something else.
ground the wire onto the tank body, the gauge should read full scale.
If not, check the +10V supply to the gauge from the voltage stabiliser, or the gauge itself.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Ok,

Well, both the temp guage and the fuel guage don't work. I've never had it hot enough though to test the temp guage properly though. So I'm now leaning towards the +10v. Where is this in a clubby? What exactly should i be looking for

There's a little tin can in behind the speedo, it takes 12V or whatever the battery/alternator dishes out and reduces it to 10V using `1950s magic' (glorified vibrating bimetal strip).
If there is 12V going in and 0V coming out, PM wombat for one of his cheap and excellent electronic ones. I run one in each car now.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.