how difficult is it to get at the speedo drive part in a Ford 4 speed gearbox mk 2 escort).

I need to remove the speedo cable as the speedo is not working (haynes says it is just a circlip holding it in place but it looks awjward to get at...).

It's an internal circlip, easy enough to remove with circlip pliers if you can get at it. Disconnect your speedo at the gauge end and pull the inner to see if it's that broken ?