I don't know, but I can speculate!
The Lycett L'Avenir appears to correspond to the Brooks 'Competition' (not to be confused with the B.17 Competition Standard). If you can figure out when they made those, I think you'll know when they made L'Avenir.
I think you're on to something. I just found this Brooks Competition for sale on ebay. It's a dead ringer for the L
ycett L'Avenir. No background info though ...
http://www.ebay.com/itm/BROOKS-COMPETITION-PEBBLE-TOP-LEATHER-SADDLE-NOS-/261828195683?hash=item3cf62d3563:g:gtgAAOSw34FVFoZ8