Might you actually be after https://selftrade.co.uk/ ?

It makes no difference. I get to the home page of selftrade, but when I click the login link I get the error message "Bad Request - Request Too Long

HTTP Error 400. The size of the request headers is too long." I repeat: this error crept in with Firefox 57 - it does not affect IE. And reverting to Firefox 56 does not make any difference. (I reverted to 56 anyway because I hate 57.)

This issue can be caused by corrupted cookies.

Clear the cache and remove the cookies from websites that cause problems via the "3-bar" Firefox menu button (Options/Preferences).

"Clear the cache:

• Options/Preferences -> Privacy & Security -> Cached Web Content: "Clear Now"

"Remove the cookies" from websites that cause problems.

• Options/Preferences -> Privacy & Security -> "Use custom settings for history" -> Cookies: "Show Cookies"

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Appearance
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Thank you for your help. I actually resolved this problem like this: First I installed Firefox 56 over Firefox 57. The problem was then that it reverted to 57 after a reboot. I then installed 57 in a different location and uninstalled 57. This is ok after a reboot, and also has cured the problem of 'too long headers'. I have done nothing to cached content or cookies. So I have solved the problem for myself because I will stay with 56. Whether this solution would solve the problem for anyone using 57 is unclear.