It has been discussed before, I believe that both Auto Addict and Taxi Driver had their rust problems repaired under the 6-years anti-perforation warranty in spite of having no recorded annual checks. Key points are:
1. Try a different dealer - some dealers do not insist on the annual inspections being stamped in the book - and that's from other memebers' experience.
2. Try contacting Vauxhall Motors directly - again from other memebers' experience VM have proven to be helpful in similar circumstances.
However, keep in mind that this is not the last hurdle to overcome - after you find a dealer that will we willing to deal with this issue under warranty, you will need to get them to agree that the actual damage is covered. Anti-perforation warranty is just that, and if the the rust did not perforate the metal they might say it is superficial and not covered...
But generally speaking it is more about finding a co-operative Vx Dealer, and they do exist... so don't give up just yet.