I have now stripped and cleaned the reserve unit.
For anyone with the same problem or just curious I submit pictures of the inner workings.
http://www.bristolcars.info/forums/a...1&d=1687089366
The top cover is an interference fit. Remove the clip holding wire to body and prise cap off along with coil. Unscrew inner part of solenoid and be prepared for Mazak sludge or for a copper ball to leap out.
There are two springs inside. One in the body of the unit that pushes on the ball. The other in the travelling part of the solenoid.
It may be possible to unsolder the brass gauze and remove in one piece to clean the pipes but there is a fair chance it will turn to dust in your hands. Replacement brass gauze / fuel pickup sheet available on Ebay if you are handy with a soldering iron for sheet metal.
The shorter tube on mine was completely blocked but cleaned out fairly easily with a welding rod flattened at one end.
So, cleaned up and reassembled. Tested with a battery. Reassuring thud from the solenoid and...
Hopeless. No discernible shut off of upper tube. I'm pretty sure the Mazak has failed between the upper end of the tubes in the casting. Time to get 4 Ashes to rebuild it with a new head.