(JAR-OPS 1.430)
Autopilot failure with an RVR less than 300m
1) Above DH: A go-around must be carried out.
2) Below DH: A go-around should be carried out. However, circumstances might dictated that landing is the safest action. i.e point at which failure occurs, visual reference or other malfunctions.
For the following I don't have JAR references, but I have taken extracts from company ops manuals.
ILS Deviation
Persistent deviation above 200RA, or any deviation below 200RA, the approach will be abandoned, unless the flight path of the A/C can be confirmed to be be satisfactory by visual reference.