On 30/07/2018 09:40, Brian Gaff wrote:
Proper oil, not wd 40. Oh and push pull the loop bit a bit and give it a
major clout. However if it does unlock, I'd most certainly replace it.
Brian

My understanding is that most of the internals are made of corrosion
free or brass parts (apart from the springs). This being the case I
doubt if it has seized, more likely to have a worn pin or key which
would prevent the key from turning.
If the key insert ok then the springs will be free and not seized.