I have always retained my certificates, right up to expiry. The last scenario went like this, I moved house and had a visit to check out the new security arrangements, all kit was stored at another address (approved) but on arranging the visit the F.O. asked for the guns to be present, so that the serial numbers may be checked with the existing (old address) certificate. No problem there, so I picked up the guns.
The visit was ok, and the serial number check took place, the F.O. then went to put my old certificate in his bag! To which I stated "I am shooting at the weekend, how will I buy cartridges?" He looked puzzled and stated that I probably wouldn't be able to, without a certificate - and promptly returned the old certificate.
My view (which is based on common sense, rather than any legal position) is that you have paid for the term of the certificate (5 years), and not 4 years 11 months - so it is your property - hang on to it!
I would never want to be deprived of shooting - or be out with the uneasy feeling mentioned earlier because of police admin - not for a single day.
If the cert has expired, that would be a different matter entirely.