Free to use, but might build my own
Whist talking with one of my sponsors, Langham Engineering, the lovely Alice piped up with, "You know we have a rolling road down at the old factory. I nearly fell off my chair!
I have now been to see the said machine and it is from the 1980's, a Souriau Perfotest Mk2 unit, several about and even video on the net of one working. It uses 2 rollers and i think air resistance from 2 fan rotors to create the inertia, it has 2 dials, one for MPH and the other for BHP, which is switchable between 0-10 bhp and 0-100bhp
Whist not ideal it will allow comparative engine testing work. I have bought an old steel framed Kawasaki GPZ500 rolling frame that i will adapt and weld stuff to so that i have a rolling test bed for the engine, nobody wanted to fit my 18 foot beauty into their dyno room, and engine dyno's still need a cradle and all the stuff mounting, and the nearest was almost 2.5 hours away.
Ultimately, having looked into it, i will probably build my own dyno, plenty of info and software about, the key issue is getting a rotor of sufficient mass/inertia to make it sensible.