Dear all, as above. Any clues much appreciated before I go on a poor-earth hunt behind the dashboard.

I solved the problem by cleaning and tightening the earth on the battery tray. God bless cars which are easy to repair...

Hello, I am reviving this thread because the problem has remained intermittent and has been getting a lot worse lately. After staring at circuit diagrams and reading the Haynes manual, etc, I am convinced it is the voltage regulator since this serves both the fuel and temp gauges. The voltage regulator for early models is shown in the Haynes manual but not for later models. After much mental effort, looking at the circuit for the instrument binnacle and web searches, I can now identify the voltage regulator for late models as shown below with a red arrow. It is an ITT TCA 700Y and it just unscrews and pulls out for replacement.
I will try swapping this component and see if it helps - a work in progress. Apparently, its a common problem in the VW community with this component failing and affecting both the fuel and temp gauges simultaneously.

Progress, potentially anyway. On removal of my instrument binnacle with the problem, I found that there were two broken connections to the voltage stabiliser so this armchair sleuthing might be paying off. Time will tell if my soldering (shown below) has fixed the problem. Optimistic, now.

I am ecstatic to report that I have solved the problem by soldering those joints. The fuel and temp gauges now work reliably all the time and even the speedo gives a steady reading after a clean-out. C'mon fans, drop us a line if you're still reading this - I am sure I am just talking to myself on here.

Good job, I bet it was exciting guessing when you needed fuel.