Thanks DaveBB,
The new bushing sent to me from Ford does not have a lip on it so I do not see what stops the whole thing moving around as the hole in the mounting is almost twice the diameter of the strut.
I've also checked the other side and it seems the shock does not come through exactly at 90 degrees, so I also do not understand what is to stop the new bushing being squished out to one side.
I'll try to install again and see if I have any better luck this time.

 

I think I see in this "how to" a lip on the bottom rubber bushing to keep it centered in the hole.
How-To: Replace your 1997-2003 F-150 2WD Front Shocks - F150online Forums
Hope this helps.

 

Thanks again DaveBB,
That is really helpful - I think the problem may be that I got an "master mechanic" to replace the bushes before and he appears to have used non-standard bushes which is causing the problem. Perhaps the only cure is to buy replacement shocks as I was told by the Ford garage that it is only possible to buy the top bushing, not the underneath top (bottom) bushing as well which seems to be the one that has the necessary "lip" on it.

 

Yes. I've had to buy the whole shock to get the proper bushings. Look at the website for energy suspension before you get the whole shock. They might have something that'll work.