No. There are conditions that require you to take reasonable steps to ensure their security. Breaching those conditions is an offence.
If the Police have reason to believe you are breaching any of your conditions, then various powers of entry would apply to allow them to secure and preserve evidence of a crime. This is nothing new.
When I was first granted a Certificate, my Firearms Enquiries Officer (FEO) specifically told me they did not do spot checks - random or otherwise. Any routine visits to do with paperwork would be by appointment.
If for some reason they did have cause to stop by - lets say another local shooter had been burgled and it looked like it was a targetted burglary specifically for the guns, then the Police might be popping round local Certificate Holders to warn them - if they couldn't get hold of you whilst they were on their way, then it may indeed end up being an "unannounced" visit.
In such a case (which has never happened, so far), he advised me to take their Warranr Cards, leave them locked outside and call the local station to confirm their identities and the legitimacy of their business. He said they would have no problem waiting for me to satisfy myself they were genuine.
This changes things very slightly in that they may now plan an unannounced visit if they have reason to be concerned about your security, but they still can't do random spot checks - and if you think that a visit is that, then it needs reporting to an NGB so it can be stamped on tout suite. Depending on the circumstances you may or may not turn them away.
If their opening line is "we're here to see your guns, let us in, we're the Police", then shut the door in their faces.
If they offer some poor excuse for an explanation, then maybe let them in for the sake of 5 minutes of your time (having independently verified their identities), but with the knowledge you are unconvinced - and then lodge their details with your NGB and complain to your Chief Constable.