“It is an enquiry relating to a particular return in respect of which a supplement may be payable if the claim to repayment is not dealt with promptly. The combination of the words ‘the raising and answering of any … enquiry’ also indicates that the word ‘enquiry’ is used in the sense of a question or questions put to the taxpayer for him to answer, not an enquiry in the sense of an investigation concluded by a report. The word ‘raising’ itself in this context is clearly used in its ordinary and natural meaning of putting an enquiry or question to, or making an enquiry of, the taxpayer about his claim for repayment.”