Reading this is really annoying , because the various Constabularies are obviously making Law to suit themselves, hence the cock ups and delays.
You do not have to send in your old certificate, you never have to surrender your old certificate.
If your circumstances have not changed since your last visit by an FLO , why can they not renew the day after banking your cheque? It only requires them to press PRINT whilst viewing your details on screen?
Also please be aware that if you are issued with a Section 7 certificate whilst awaiting a delayed renewal , it only allows you to secure your gun at home , it is not a certificate to use your gun or buy cartridges. Many forces do not like to issue Sect 7's as it is traceable by the Home Office and can be considered a measure of the Forces inefficiency. If they can issue a Sect 7 , they could have issued a renewal.