Official Honda helpline told me they do not have a database of the codes, only the local dealership does, and it is up to the dealership if they charge you for the service or not.
I then remembered that there was an MOT reminder sticker in the window, and so called that dealership to see if they had the code.
The very nice man in the service dept explained that if the car wasn't sold at their showroom, I would have to take the car in to them so that a) that I'm not just holding a stolen radio unit, and b) they link it to their computer, get a serial number (apparently not the one I had noted down already, he clarified that) and send it to Honda, who would return to them with the code.
Fortunately he checked the reg plate and it was sold by them, so within seconds I had the code to unlock the radio
Greenacre Honda Clitheroe, I salute you.