The sticker on my C5 had also faded badly, but I managed to work out the number using this "bright light / shallow angle of view" method! It took a couple of different lights, and a camera flash, but it's not too difficult.
Also, the number on the sticker isn't a sequential in terms of total
volume production, but rather the month, year and the production
within the listed month.
Taken from the excellent resource at
www.sinclairc5.com:
What does the Vehicle Number mean?
V100050302778
V1000 = Model reference for C5
5 =1985
03 = March
02778 = 2778th produced that month
Yours looks like it starts:
V1000 5 10 0...
So, at least you know it was built in October 1985