153361, it's actually an additional unit! 318 was supposed to be going to Northern but never went!
It depends what you mean by "additional." Both 153318 and 153361 appear on Tom Cairns "West Operational Fleet" list :
Class 153 (12)
153305
153318
153329
153361
153368
153369
153370
153372
153373
153377
153380
153382
Its "additional" in the sense that
FGW▸ have 2 more Class 153 units than what was set out in the Franchise Agreement, but not "additional" in the sense that FGW have 3 less Class 153 units than Wessex had.
Also, 1 of those units is being retained for use on future extra Falmouth line services (covering for refresh in the meantime.) The Falmouth initiative is likely to be funded by a mixture of Cornwall County Council and
EU» money.