Thanks for the replies chaps.
I'll call Clove tomorrow cos it did it again last night. The fact the "torch" won't come on after cut-out, & I cannot check the remaining charge in the iwalk says something must be wrong.
I am still puzzled why they say on the instruction leaflet that you may need to charge it a bit to get it to come back to life... why would I carry the charger for the iwalk if I'm expecting 3 full HD2 recharges out of it? Doh!