Most LEL literature seems to refer to Lee Valley.
The YH website is for Lee Valley YH.
Odd, because its by the River Lea.
Googling this knotty problem gets lots of hits.
Apparently there was an Act of Parliament back in, oh, 1760-ish, to enable the construction of the Lee Navigation (the canalised parts of the River Lea).
The 2012 Olympics are to be held in the Lea Valley.
The OS maps hedge their bets, marking a car park in Luton as the 'Source of River Lea or Lee' and staying with this convention all the way to its meeting with the Thames. However its the Lea Valley Walk, and the Lee Valley Park.
Just thought you ought to know.