I would submit a complaint here and wait 2 weeks for their feedback:
https://secure.cic.gc.ca/feedback-retroaction/feedback-retroaction-eng.aspx
If nothing happens, I would send a formal complaint to the London VO email (if prepared by an immigration attorney, even better). 18 months is excessive, specially when you don't have an explanation for it.
Even if it delays the application a couple of weeks, I would prefer to know what's going on. And hopefully that will bring attention to the application.
(My CSQ application took twice the normal processing time, and 2 weeks after a formal complaint sent by my immigration lawyer, it was all sorted out and the CSQ delivered. My CSQ application was apparently stuck due to a payment software problem they had at the time in Paris VO, so they thought I hadn't paid the fees.)