I've come across a huge fare differential among the top travel sites, and was wondering what the deal is--

I'm looking for YUL-BEY, with a stop in london. opodo.co.uk is showing LHR-BEY for £396.50 on OS, but no other site has this fare, even the OS website. is this still a published fare that my travel agent could book, or am I stuck to book with opodo or nothing?