We often hear the word 'fortnight' on British TV imports.
Does 'sennight' have any currency there? (it's marked archaic in MWC)

http://members.aol.com/tsuwm/

"Fortnight" is in regular usage in New Zealand, probably because of the country's historical (and therefore linguistic) connection with England.

But that denies the US historical (and therefore linguistic) connection with England.

Am I opening another can of worms?

Fortnight is certainly in common usage in my experience in England but sennight I've never heard used in my 40 years living in the South East, the Midlands and West Yorkshire. It may be used in a region I'm not familiar with (regional variations can be quite large and Scots is a seperate English dialect).

Fowler says -
sennight. So written; but the word is now almost out
of use. It might have been, but will now hardly be,
utilized as a substitute for "week" in the formula "on
Monday &c. week" when the reckoning was to be
backwards: We start tomorrow week, BUT It happened
yesterday sennight.

Brewer says, inter alia, that se’nnight and fort’night are relics of the ancient Celtic custom of beginning the day at sunset … also ‘The evening and the morning were the first day’ (Genesis).