PAXboy - yes, your understanding is correct. The cost of an extra one-way from Heathrow to Paris booked months in advance minus the refund of taxes is small, compared to the potential cost to me (not just monetary) of not catching the onward long haul flight. The later flight to Paris would lead to a tight connection in Paris with two airlines on different tickets. The risk around self-connecting and the possibility of flight delays makes it possibly rational to buy a 2nd short-haul ticket at an earlier time as a form of risk-reducing insurance.
edi_local - many thanks for your very informative post