I'm looking for a way to treat cracked leather.

The leather uppers of my Scarpa hiking boots are starting to develop
cracks at the sides, where the toes hinge. This is despite regular
treatment with Nikwax-type products.

Googling returns leather repair products, such as this:
http://preview.tinyurl.com/k7t3npv
Most products seem to be aimed at upholstery repairs and user feedback
is mainly about cosmetic aspects rather than durability.

The boots have completed around 1400 miles over rough terrain in nearly
3 years and the soles are fairly worn. I think they're usable for a
while longer, so I'm looking to eke out a few more miles, but expensive
repairs would be uneconomic.

Any thoughts/experiences/recommendations for a repair e.g. filler or
coating product, whether specifically designed for leather or other? Car
body filler probably wouldn't flex enough :-)