As ever, it would pay to read the source rather than an interpretation. Article 50 is readily available online, clearly written and very short.
And article 50.3 says
. The Treaties shall cease to apply to the State in question from the date of entry into force of the withdrawal agreement or, failing that, two years after the notification referred to in paragraph 2,
unless the European Council, in agreement with the Member State concerned, unanimously decides to extend this period.
Jeremy - there is a difference between precision and pedantry.
Given the disarray and split both between and within the various parties in parliament, it is inconceivable, today, that such an extension could/would be proposed/considered without it being at the request of the UK Government - IMO.
What steps and hoops UK GOV would need to go through to get there, is another question - one, that perhaps, you could enlighten us on.
Edit: added ‘IMO’ for clarity.