Turns out that the £334 quote I mention in my original post was with the maximum of 6 mods you're allowed to enter on any online quotation site.
When trying to contact the insurers directly to declare my other (relatively minor) mods, they refused to insure me as the 'total mod count' was over the magic number of 6.
Sky Insurance came to the rescue and managed to get me, my car and my mods fully covered for £522.
Moral of the story is:
It's cheaper (for me at least), to insure an unmodified Lambo Gallardo than it is to insure a modified 350Z.