Eventually realised the problem was due to instability in the voltage regulator.
The load capacitor (C2) should be 8 - 10 times the size of the input capacitor (C1), on the ME board both capacitors are the same size (100n). Swapping C2 for a 10u cap fixes the problem. The selection seems to be tolerated on an MCP201 but cause big poblems on a MCP2021.
ME LIN board reference : http://www.mikroe.com/eng/downloads/get ... l_v100.pdf
MCP201 datasheet : http://ww1.microchip.com/downloads/en/D ... 21730F.pdf