Bienvenu Aurelien,
Your Morris C4 was built as a "Truck, 15cwt, 4x2, Light Warning" which means it would have had a van body similar to a wireless truck, which is why it has got the letters WT in its chassis number. Its Army number was somewhere between Z5832027 and Z5832356. I can't give you a build date, sorry.
There are pictures of Morris WT vehicles in post number 20 of this thread