According to the internet, the MT75 has an unusual synchro hub arrangement on 1st/2nd gears.
Vehicles manufactured from build date
17. 6.1996 are fitted with a 1st/2nd gear
double synchroniser unit

Look at page 00-01-13 here...........
http://www.fordscorpio.co.uk/manual/manualtrans/MT75Overhaul.pdf

If it engages OK with the engine off, then the problem is either with the synchro hub or tickover speed is too fast (this wouldn't be as big a factor for 2nd gear.
See if there's any difference in engaging 1st gear with a cold engine (fast tickover) and a hot engine (normal tickover).

If it's harder with fast tickover then it's either the synchro hub, or clutch drag. Given that the remaining gears seem OK, the former seems more likely.

DaveW
'05 Red Roadster S1
'16 Yellow (Not the only) Narrow AR GDI Plus 4