Problems with the “The Unusual Betrayal” campaign.

I'm having some problems with the "The Unusual Betrayal" scenario (scenario 5) in the Haven campaign, where I need to defeat Sir Landrew. I've already captured all the towns, mines, and buildings. I've explored every possible corner, but the scenario won't end. The task is to destroy Sir Landrew. I know he's blue – I captured a blue castle – but there was no Sir Landrew on the territory of the blue faction. If anyone has encountered this problem or knows how to solve it, please help!