It's a miracle baby.
Based on information provided:...
Lmp 5/12
nmp 2/01
approx 28 day cycle
expected ovulation date approx 19/12.
Sperm can live upto 5days in a fertile woman
the egg is viable for approx 36 hours after ovulatuion.
Puts your high fertility window at 14/12 - 21/12
sex on 29/12 is too late to get pregnant (even with late ovulation) and get a positive test with such a high beta on 7/01 so that rules the new man out.
However, sex on 10/12 is too early in all liklihood.
Are you sure you didn't have sex again with your ex again after 10/12?
If you're sure then it looks like you are an early ovulator, in which case it's your ex's - the beta would indicate this to be the most likely outcome also.
If by some chance you were a late ovulator with a short luteal phase, then I would have expected a beta in the region of less than 500...
Either man is unlikely but your new man is very improbable.
Based on your info given and typical/likely cycles i'd have to say it is your ex's baby - i'd be very very shocked to learn otherwise.