SO I changed the WE to AI from the Watcheye so now OpenCPN receives !AIVDM (checked it) and same result - no CPA or intermittent CPA (on my Win10 machine the AIS details window shows the CPA/TCPA a second or so every 10 or 15 seconds). But the alarms work great (in fact, the bell is driving me mad as I type this).
Now here's the thing: If the MUX sends !SHxxx from the Standard Horizon, OpenCPN works perfectly. If it sends !AIxxx from the Watcheye it fails, as it does with !WE (no difference).
My
Raymarine C120W works perfectly whatever ID or Source I choose (Standard Horizon or Watcheye, SH, WE or AI). It's like OpenCPN trips over something coming from the Watcheye and as the inside of the Watcheye is from the same manufacturer in most brands of AIS I am guessing this problem is more common that it may seem?
It is not a major problem for me, having two AIS receivers. But I choose to have two so I have a fallback so I prefer to have it working in OpenCPN.