if your tax is £30 it must be an 82 or a later 86/88 engine all the others are £105 tax ...sorry..."road fund licence" 
if you put your reg into mycarcheck (you don't have to pay) it should say at the top a Nissan Micra (spec name) Dci 82 (3/5 door Hatchback)
my sport+ was advertised as an 86...but I still double checked and was pleased that she was, in fact, an 82 ^_^
...........the 82 engine is 119g CO2/KM band C
The later 86 engine is 120g CO2/KM also band C
http://www.parkers.co.uk/cars/reviews/facts-and-figures/nissan/micra/hatchback-2003/running-costs/
it should also say this on your v5c
