Giving a coulpe of goes on the heaters is a good idea on these engines at this time of year.
What battery did you fit, as some listings for the diesel range on escorts list a std petrol battery, and this simply is not man enough, had the same on a escort I bought years ago, correct battery according to the chart, but nowhere near man enough.
You could try cleaning the connections on battery and starter, also the earth points, particularly the one on the engine.
If it is purely slow turning over but only in cold most likely battery related, next time just connect up a boost pack to see if that changes things, if it's then perfect you know it is down to batt being a touch small, even though may well be what they recommend.
If you have a really heavy grade oil in the car this can also do it/make it worse.
tabetha