Just read the threads on this topic and find it unsurprising the amount which has been quoted bearing in mind age, no ncd and insurance group. I'm afraid that even though you and others of your age group within the club can be classified as safe, competent drivers, you are paying the price for the apparent recklessness of others in your age group who are not as careful and are involved in more accidents thereby loading the premiums. You might be able to shave a few hundred quid off but I doubt it.
On the subject of modifications and advising the insurance company, am I right in thinking that the insurance company need only be advised if the engine performance of your car has been improved beyond the power output expressed on the registration document? I know many, including myself, have bolted on goodies which under normal circumstances should increase power output but because of the Nissan ECU such performance increases are generally negated. Therefore, if you can demonstrate through a dyno read-out that engine power output has not increased you need not advise your insurance company? I know they use the phrase 'modified' as a catch all but upgrading to 19" wheels or fitting better discs etc., should not increase premiums per se.