Just been nosing around London TravelWatch's webpage for Oyster on the
rails & the last para here -
http://www.londontravelwatch.org.uk/news.php?id=696, says

"Oyster Extension Permits

If your Travelcard is on Oyster, and you wish to travel outside your
zones, but still within London by national rail, you will now be
required to ??~set’ an Oyster Extension Permit before the start of
your journey at a ticket office or machine. If you do not do this, you
will liable for a penalty fare of £20, even if you have ample credit
on your card. If you have an annual ticket (Gold Card) it will be
cheaper (and equally as inconvenient) to get a paper extension, as you
need to currently"

OK, so please explain that last sentence.
"If you have an annual ticket (Gold Card) it will be cheaper (and
equally as inconvenient) to get a paper extension, as you need to
currently"