‘If a person who, apart from this sub-section, would be liable to a surcharge under sub-section (4) above, satisfied the Commissioners or, on appeal, a Tribunal that, in the case of a default which is material to the surcharge : (a) the return or, as the case may be, the VAT showed on the return was despatched at such a time and in such a manner that it was reasonable to expect that it would be received by the Commissioners within the appropriate time limit he shall not be liable for the surcharge and for the purposes of the proceedings of this section he shall be treated as not having been in default in respect of the prescribed accounting period in question …’