Ok cheers. I think the only spare LCD I have is one from the official Arduino Starter Kit so most likely a 0x27, which I will initially test your Firmware with. I don't want to pull my other project apart to get the 0x3F board out.
I guess it is something for people to note if their LCD does not work. I have a cheap Chinese order on the way, so I will test that in September.
I found a build pic of the 0x3F LCD:
