Well that's a roll of film wasted then! Why would the manufacturer state 20-40 seconds is beyond me. I wanted to stick to the recommended time as I read too much fixing ruins a film in several ways.

I do have an all in one flat-bed-scanner, a recent one, it can't scan film but surely I could scan it and invert it with software.

Update: I tried it with a colour negative there and it has a very blue hue.

Mr. Rusty, I measured my proposed space there, 1.6m x 0.76m with a sloping roof for the stairs (this is no good)