Ford Escort 75 1.8D Van N Reg?

Drove Van started fine when i returned and tried to start it, it wont start wont turn over. Have changed the starter motor, and had the battery checked all ok. Have been told it may be the immoboliser which could prove a problem as my key doesnt have the usual button on it. Has anyone got any other ideas of what it might be or how i can sort this out myself, if i have to take it to a garage i might as well scrap it because ive been told its quite expensive. Other than this problem its been a good van.