Just to update this...
Since my first post and having removed the CEM completely, split it and gently filed the pins with the wife's foam backed nail file and then cleaned the board, the pins and the multi-plugs (again) with contact cleaner and a toothbrush, reassembled, cleared the codes... a couple of months and about 5,000 miles have passed and thus far, it's been fine with no CEM related issues to report.
Granted, I've probably now jinxed it and it'll pack in tomorrow but given it was giving issues on a fairly regular basis, whilst I can't say it's fixed fixed, it certainly appears better. Whether it was filing the pins or another clean or the plugs / pins are sitting better or all of the above, who knows...
Until it happens again, I'm not inclined to start fiddling with it again.
Watch this space...
