I've only heard it repeat the platform at the end in the 'the xx;xx xxx service to xxx will now depart from platform x' rather than in the format mentioned above.
The other style (which SWT use) is:
'Attention, please: this is a platform alteration. The xx;xx xxx service to xxx will now depart from platform x. Platform x for the xx;xx xxx service to xxx' for the first announcement to denote the platform alteration, then they use
'Attention, please: this is a platform alteration. The xx;xx xxx service to xxx will now depart from platform x. Calling at: ... . the xx;xx xxx service to xxx will now depart from platform x' as the second announcement.
(apologies for all the x's)