Hi Paul and welcome to Eribafolk.
The link that Hob has given you in Post 2 is useful, so just to reiterate:-
If your Eriba has a 6 digit number starting with 5 it is a 1980's. 51 will be 1981, 52 will be 1982 etc
If your Eriba has a 6 digit number starting with 6 it is a 1990's. 61 will be 1991, 62 will be 1992 etc
If it's a 2000, 2001 or 2002 but NOT a Touring GT then the serial number will start 6100, 6101, 6102.
If you've got a GT model then 2000 is 6200, 2001 is 6201 etc.
However if your Eriba is earlier than 1980's then although we can assume that it should start with a 4, we are not sure. The best thing to do in that case is to get the serial number off the plate and phone Jandi http://www.eribacaravans.co.uk/ (Jean or Sandy) who will be able to help you.
Hope that's of some help.
Randa