“The Code (i.e. part X of TMA) does not qualify the expression ‘reasonable excuse’ by, for example, ruling out reliance on another to perform a task such as making a tax return. The obligation to make the tax return on time is nonetheless the taxpayer’s. It remains his obligation regardless of the fact that he may have delegated the task of making the return to his agent. There may be circumstances in which the taxpayer’s failure, through his agent, to comply with, e.g., the obligation to make the return on time can amount to a ‘reasonable excuse’. To be such circumstances it must be something outside the control of the taxpayer and his agent or something that could not reasonably have been foreseen. It must be something exceptional.”