what is different about the Zetec E engine ?? and how can you tell if you have one ??

Compared to what?

compared to the zetec I have seen both types referred to is it one and the same or is one restricted by throttle body etc ??

The Zetec E is the standard iron blocked 1.6, 1.8 and 2.0 as fitted to Escort/Focus/Mondeo.

Are you perhaps thinking of the alloy blocked Zetec SE (1.25, 1.4 and 1.6L)?

I am after a timing belt and have seen a supplier advertising belts for the 1.8 zetec and the 1.8 zetec e

This is wher my confusion lies. I have recently purchased a 1.8 from a 1998 mondeo so would any belt for a 1.8 silvertop fit ???

The zetec E was called zeta engine in the very beginning and was a collaboration with mazda, then got a name change to zetec E. Both blacktops and silvertops are zetec E.
I don't think the term zetec without the E is valid. Too confusing when ford also use it like a delux badge on the tailgate of cars that don't have any sort of zetec engine in them. Zetec SE is a collaboration with Yamaha and is a completely different engine.
Changeover date for silver to black is mid 98 so be sure which engine you have. One belt does all 1800/2000 silvertops from whatever car.