There is no legal requirement to have your shotgun or firearm certificate with you when you are out shooting.
However it is sensible to do so in case you are stopped by the police for any reason.
They have the authority to seize guns if the owner is unable to produce his certificate.
In practice they should never need to do this as computerisation and modern communications techniques mean they can easily check with HQ, even out of working hours, whether or not you are a certificate holder.
The only time you need to carry your certificate is when you are buying ammunition, getting your gun serviced and the like.
Your mates owe you a beer!