been to shell island loads - for starters, forget the official site, it's rarely updated, and tells you very little. try this link for a much better idea of what it is like;
http://www.shellislandcampers.co.uk/index.php
i'll try to give you an idea of what it offers;
first, the place is massive - pitches range from big fields that you share with loads of other tents, to tiny little enclosed pitches with room for one tent. my advice is to spend a bit of time driving round the island to find a spot that you like. (and don't just stick to the roads - there are loads of good places that you can't see from them)
if you want the best choice of the private little pitches, then don't go at peak times - school hols etc.
you can build fires anywhere on site, as long as they are off the ground and contained - firepit etc
there are miles off beaches - good for foraging - shellfish etc
any type of tent is good - bivvys, tipis, anything - if you want to sleep under it, then you can!
the 20-yard pitching rule is well observed - but if you go as a group with more than one tent, you can all pitch close together in a circle.
despite what you may read on other forums, the facilities are fine - if you want marble sinks and shiny taps, then camping really isn't for you - i tend to think that negative comments made about the facilities are from the type of campers who like to pitch around the edge of a square field and plug their tv into the electric post behind the tent!!
miles of sand dunes - plenty of maram grass etc - good for tinder
in short, shell is easily the best campsite in the country, and as close as you can get to camping in the wild without the risk of coming back to find the contents of your camp have been nicked (an experience i'd rather forget!)
get in touch if there's anything else you want to know!