Finally tracked down an issue of intermittent communications between a Nano EVERY controller board an a MKR CAN shield.
The Nano EVERY controller, like almost all Arduino controller boards, uses +5V signalling on the SPI bus, whereas the MKR CAN shield has it's CAN controller SPI chip (MCP2515) powered from the +3.3V rail. This makes the SPI bus signals close, but not guaranteed to meet the correct input voltages for ViH back to the controller and in fact, overdrives the inputs into the MPC2515 chip (possibly damaging it).
I modified the MKR CAN shield to strap +5V power to the MCP2515 chip (which can take 2.7V to 5.5V Vcc) and now it works 100%.
Heads up to all.....AA