Hi Shygirl. I have a 2 year old Ifor Williams BV125-7, which has the same hitch I think. It is certainly a tiny key and it must have seized the shaft that locks the release handle, rather than the lock itself if WD40 won't work. I use spray chain lube as it goes on thin and penetrates, then the solvent evaporates and it leaves a good thick coating. I've just looked at mine and it is very difficult to get to. The only thing I can suggest is removing the whole coupler assembly from the end of the braking shaft, turning it upside down and flooding the thing from underneath. I suspect mine will go the same way as yours as it all looks very dry under there.
It's just occurred to me that perhaps the bolt can't be removed, or if it is cannot be replaced. Perhaps a call to a device centre would be an idea?