cold spell starter problems help please

[Gil]

hi , i have an old j reg transit lwb sht 190d it has only got 56000 on the clock and is in good nick.2 or 3 times during this cold wether she would not start and was not easy even with a jump start,she has got two batteries.i took her to the garage expecting it to need a battery replaced but the mechanic tested it a few times and said the batteries were great it is an alternator problem.i have been trying to jump start it at the moment with no luck .any help or suggestions welcome please.thank you.

[MrMPuk]

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