Poor old Newton.  Afraid my first thought is: ‘simple answers and someone to
blame’.

I would argue that seigniorage is the primary matter here, as I think the
flight of silver appeared immediately seigniorage (at c. 6%) was abolished,
in 1666, and more importantly, because the problem was solved by the
reintroduction of seigniorage on silver (at c. 6%) in 1816.  The
reintroduction was camouflaged, at the time and ever since, by calling the
post 1816 silver coinage “tokens” – but their intrinsic launch value was in
fact the traditional sterling one.

Of course the respective gold and silver buy prices at the mint played an
important role, but those were not necessarily fixed in stone by Newton. 
The problem ran on for 150 years, and a failure of remedial action over 7 or
8 generations of the governing class was involved.  Nor is it obvious to me
that it would have been a good idea for the government to start intervening
in the international bullion markets to maintain its currencies.  Gordon
Brown perhaps demonstrated recently that politicians are not too good at
that game?

Surely the real story here is that Britons played a very skilful monetary
hand internationally in the 18th and 19th centuries, but that a rather heavy
price was paid domestically for this international success, by most of the
population, prior to 1816?

Getting back to Newton, I have not noticed anything dramatic happening to
silver coinage in 1717.  Rather I see it gradually slowing, until issue
finally stopped in 1758.  What did happen in 1717 however was the
re-introduction of copper coin.  Nick Mayhew recently pointed out that the
rather copious copper issues, 1717-1754, importantly plugged part of the gap
left by disappearing silver.   So there is a case to be made that good old
Isaac was not after all the psychopathic dyslexic that the post modernists
at the BBC these days seem to want to paint.  Rather the very opposite, one
man who, monetarily, stepped up to the mark, in a century where almost all
others failed.

Rob Tye, York, UK