Good news. It's working now. I think it was more a problem with my computer than with anything else.
I tried installing AST to a different directory away from SE4. I tested SE4 to make sure it would work OK without AST and there was the same load error. This is wierd because AST was not associated with SE4 in any way this time. I'm thinking that my computer mustn't like Winzip or something else has caused a corrupt.
I re-installed the game and then installed AST to a different directory. I restarted the computer to get rid of any corruption and this time I just moved AST manually across to the SE4 directory. Via, we're in business!
I don't know why. But I'm pretty happy. I'm sure it isn't any problem with AST - just my computer.
Thanks for your help, Atrocities.