I have oil fire heating in my new (quite old though) house. Its in a recess
and Ive just put a door round the oil heater but someone has worried me and
told me that they are not supposed to be enclosed and that I should have put
some kind of ventilation in rather than souround it  with doors. Is this
true - should I take it off?

Try:-

http://www.oftec.co.uk/customers/customers.htm