Any recent pics Andy? Haven't seen it for a while.
The gauge will be a bad earth. I had the same problem with a water temp gauge a while back.
To test the sender. Put a multimeter on the terminals and check there's some resistance there. Ususally as the sender gets warmer, the resistance decreases. So if there's no resistance, the gauge will go straight to max.
Hope that makes a bit of sense.
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My Build Diary -
http://davesroadster.blogspot.com/
*updated 7/6/11*
Best bit so far: Getting it running
Worst bit so far: Cutting steel for wishbones and engine mounts.
Most annoying bit so far: Generator packing up half way through welding.