Polo 1.0 (6N) 16684 & 16688 Misfire Codes

[½cwt]

I've just picked up a cheap (yeah, usually a good reason...) 2000MY Polo 1.0 for my step-son to learn to drive/get road experience in.

Having replaced the battery so it actually starts I've got two sets of fault codes, 16684 and 16688 on the engine (random multiple mis-fires and cyl 4 mis-fire) I've changed cyl 4 plug lead for a known good one but still have the problem. It is an intermittent fault, usually under throttle applications between 1900 and 3000 rpm, plus idle is a bit lumpy occasionally. Where to look next?

The other set is 00290 and 00287, both for left rear wheel speed sensor. No ABS light at the moment, most info seems to be a good clean around the sensor as the best start.

Chris