gearbox 200tdi problem

Has anyone got any ideas to this problem. I have had a second hand landrover genuine recon main gearbox fitted today which i supplied from another 4x4 supplier fitted by my local well known 4x4 place. when tbox they fitted a new clutch complete and mainshaft gear t/box and main mainshaft bearing. all sundires were changed ie pushrod to clutch, spigot bush etc ie all works carried out properly. my problem is the gearbox supplied was sold as all gears change well, drive spigot good. the 4x4 place that fitted it have reported to me reverse gear difficult to engage, 1st and 2nd ok and 3rd and 4th difficult, 5th good. on driving it myself found gearstick very tight like a new vehicle but difficult to select gears as above. on driving vehicle for aprox 50miles diliberatly changing from 3rd to 4th reguarly some improvement has been noticed. i contacted the supplier of gearbox and found out box had been out of last vehicle aprox 1year. does anyone out there know is it likely to loosen up or is there a fault? obviously it is my responsibility because i supplied the box to the garage who fitted it. any help much appreciated

cheers
bob

have they used the correct oil mate? it has to be either MTF94 which as i found is hard to get
or ATF fluid derox 2 or derox 3