I cannot really answer this is as it totally depends upon what the letter of intent says i.e. does it refer to the ECC form ? If 'Yes', what data is filled in the Contract Data, what is in the Works Information etc etc ? And in particular to your query, what (if any) timescale for submitting a first programme from the Contract Date (not starting date) is filled in the Contract Data as referred to by clause 31.1 ?
As one lawyer says, letters of intent are often letters of no intent due to their vagueness, but they are contracts in themselves. Their vagueness helps him make a living !