The offence is not being in posession of a valid FAC. I know people who have been given custodial sentences for posessing fired shotgun cartridges without a valid certificate, even though they HAD a cert, they didn't have it with them!
I will say it again. NO MATTER WHAT THEY HAVE TOLD YOU, WHETHER IT'S ALL RECORDED (on another point, did you tell them they were being recorded? If not then you have committed another offence, and certainly wouldn't be able to use it as evidence in court!) OR NOT, UNLESS YOU HAVE IT IN WRITING, THEY CAN, AND WILL, STILL ARREST YOU FOR POSESSION WITHOUT A VALID CERTIFICATE! DON'T BELIEVE THE WORD OF SOMEONE AT THE END OF THE PHONE, THE LEGISLATION IS VERY CLEAR, AND THERE IS NO ROOM FOR MANOUVER!
YOU MIGHT WELL GET AWAY WITH IT, BUT IF YOU DO IT'S JUST LUCK!
Sorry for the shouting, but I agree with central, is it worth the risk? Get in the car and drive there, sit in the front office until they hand you either a valid cert, or a section 7 permit, or you are NOT covered and could face big problems, from the loss of your FAC (permanently) to a lengthy prison sentence!
The law doesn't leave room for common sense in this case!