Forget trawling through Act and other legal docs.
The GEN section of the AIP (for any country) will have a section that clearly lays dow the requirments. The UK is no different.
Read the GEN section ad you will not go wrong.
Regards,
DFC
Unfortunately that's not the case -
UK AIP GEN 1.2 and 1.3 provide no clarification additional to that available in the GAR form guidance notes, and in fact has less detail.