Euro car parks parking fine

So I need some advice on a parking fine I received. I work night shift in Glasgow and the day we were going home I parked it at the tobery carvery at 18:43 and left at 3:32 as it states on the letter. I received the letter a week ago and have till the 29th to pay.

They was no sign to indicate I couldn’t park there for free so I assumed I could. Do I need to pay this fine because a lot of threads online are telling me not to but I don’t want it to bite me later on I.e court case lmao.

Any advice would be appreciated.