The other thing to bare in mind is that your Mini is 30 years old now so may well have had panels replaced in the past.
If the front end has been renewed, the fitter may not have taken the time needed to align the panels well which will always result in the panel gaps and fit being poor.
As Matt has said though, the bonnet looks to be the correct type although it does appear someone has cut a bit out to make a vent although obviously that is nothing to do with the fit.
There isn't a huge amount you can do really without cutting it about although you may be able to achieve a better fit by playing about with the alignment of the bonnet.
Ben