I don't understand why they want the announcement to be played as the train arrives. It's no use to anyone on the train, nor much use to people on the platform.
The best method in my opinion would be something like:
2-3 minutes before arrival/whenever the platform is clear if there is a train in front:
"The next train at platform 4 is the 12:00 First Capital Connect service to Sutton, via Mitcham Junction. Calling at Elephant & Castle, Loughborough Junction, Herne Hill, Tulse Hill, Streatham, Mitcham Eastfields, Mitcham Junction, Hackbridge, Carshalton and Sutton. Customers for stations to Beckenham Junction and West Croydon should change at Tulse Hill. This train is formed of 4 coaches. The next train at platform 4 is the 12:00 First Capital Connect service to Sutton, via Mitcham Junction."
Train entering track circuit:
"The train now approaching platform 4 is the 12:00 First Capital Connect service to Sutton, via Mitcham Junction. This train is the service from St Albans. Please allow customers off the train first."
Train standing at station with doors open (achieved by timer):
"London Blackfriars, this is London Blackfriars. Mind the gap between the train and the platform please, mind the gap. The train now at platform 4 is the 12:00 First Capital Connect service to Sutton, via Mitcham Junction. Calling at Elephant & Castle, Loughborough Junction, Herne Hill, Tulse Hill, Streatham, Mitcham Eastfields, Mitcham Junction, Hackbridge, Carshalton and Sutton."