Chaps,
All good advice, but did it answer the question that was asked?
AIP 1.5- 32 7.3 (Jepp Terminal 2.7.3.1) Altimeter Checks is the only AIP/Jepp reference to an actual tolerance in the IAL/missed approach.
The only other time navaid tolerances are mentioned in the AIP or Jepp books is in relation to (1) Aircraft Off-Track in CTA - Advice to ATC (Jepp ATC 5.4.6.2) and (2) Avoiding CTA (Jepp ATC 5.5.2).
These sections are often remembered as the 'tolerances' for other stages of IF track keeping.
They are pretty close, but not definitive.
The correct reference for tracking tolerances in instrument approaches comes from CAO 40.2.1 Appendix I, the Instrument Rating Test.
3 FLIGHT TOLERANCES
3.1 Flight within the tolerances specified is necessary for the applicant to be judged proficient in the required flying manoeuvres. There shall be no sustained errors in excess of the specified tolerances.
[snip by ITCZ]
3.5 Navigation aid tolerances:
[snip by ITCZ]
(c) ILS and LLZ ± 1/2 scale deflection (and equivalent on expanded scale); able to land from minimum altitude without undue manoeuvring;
You could argue that a tracking excursion outside half-scale deflection, that has been noted and a correction applied, is allowed and executing a missed approach is not mandatory. Exceed full scale in any way, shape, or form, and you must discontinue.
Your ops manual (and your personal standards) might apply tighter tolerances.
But you asked what the rulebooks said!