Some Merseyrail staff do fine people for smoking in shelters/canopies marked with no smoking signs but like most stations if there's just one member of station staff there and a group of people smoking they probably won't confront them.
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The law states smoking is illegal in any enclosed public space. This means smoking in ticket offices, waiting rooms or trains is definetly illegal. Canopies and shelters are a bit debateable though but most transport operators and councils have decided canopies and shelters are enclosed public spaces.
There's no fixed fine for breaching a No Smoking rule. Fines, in terms of Penalty Notices for Disorder (PND) or Fixed Penalty Notices (FPN), certainly as far as the railway is concerned, aren't used for byelaw offences, but are reserved for recordable offences. Therefore, Merseyrail staff would have to report for summons instead.
Also, as long as you can prove, either through the offender's own admittance, or through obtaining CCTV evidence etc, that they passed a No Smoking sign, then they can be reported. They don't have to be particularly near one. Bear in mind, where the Smoking ban exists, TOCs are only obliged to put up signage at each entrance/exit to the station for it to be enforceable.