After a bit of reading and consideration I decided to clean the fuel tank myself. I soaked  it for 12 hours with 12:1 hydrochloric acid and then rinsed it with bicarb, then lots of water.  It looked pretty much spotless inside so I reinstalled it complete with new hoses.  

To prevent further mischief I have also installed the biggest fuel filter I could find between the tank and the fuel pump.  I have also temporarily left one of the smaller inline filters in place.  It will come out in a little while.  I have opted to use a new aftermarket pump rather than the original SU pump, cheaper, more robust and more reliable.