I agree that this sounds a bit too beuraucratic if you already proved your permission just a year ago, but the police will have taken advice from their police solicitor (who as a race hate risk!) and will have been told that checking the permission again is necessary because the permission could have been taken away since your FAC was granted. Obviously, this is crazy because permissions come and go, and nowhere do the Firearm acts require you to show that you have good reason to possess the firearms at every single minute that you hold the FAC. I expect many people will have not had formal permission to shoot at one time or another, especially if you buy shooting on a daily basis, the point is you need to show good reason to get the FAC in the first place and generally need to be able to show that you have both a reasonable expectation of going shooting (the intention) and the reasonable means of doing so (the ability).
Trouble is, if you don't produce the paperwork, at best you'll delay the application and at worst, you'll be branded as uncooperative which may be noted on your file. Once again, the legitimate firearms owner has to jump through hoops not intended under the original legislation.
Make sure that when you get written permission this time that it states that "Mike R has permission to use any firearm on land at X", don't specify the calibres. If they insist on that, then make is say ".22LR, .17HMR, .243 and .308 rifles and such other firearms held by Mike R from time to time".
Oh, and a final thought: take a bottle with you when you go to ask the landowner!