Slightly off the subject but useful to know.. . . . .
After speakin to the good people where I get my air fills, Aqualogistics in Stockport who specialise in bottle testing.
The story of getting a DOT bottle HSE tested is this.... call them with the make and model of the bottle (which should be printed on it) they'll be able to tell/find out for you whether the DOT bottle can be tested to HSE level, so if it is then they'll do it for you!
They'll also test any other bottle you've got as well!!
Give them a call on 0161 480 5745 and they'll sort you out!
steve