You passed your city and guild in 1976 and wish to get a copy of the certificate how do you do this?

I attended Northampton college i 1975 and 1976 on a brick laying course, i passed my city and guilds i was there on a part time basis. while working for a company called Winsor& Cleave Ltd, Peel Road, Wolverhampton, Bucks, I am now required by law to prove my qualifications in order to continue to work, So i need a copy ofmy city and guilds, a few years a go i contacted the collede for this and they said a lot of this information had been lost in the transfering of data to computer from files,but i am sure there must be a record of these some where so would like to know where and how i go about obtaining them.