The safrane tank strap isn't too expensive to be honest, I think SAFRANEV6RXE said about £15? (and yes I will be coming back to you about that in the future when things improve!). The tank may sag down a bit but it will not drop off, the rear subframe ensures that. When I broke up a safrane last year getting the tank out was a royal pain in the grass and even with the sub frame down it was a menace to get out. While it's safely held in place, the rotted strap would probably still fail the MOT I suspect.
As for the smell on the passenger side, have you checked out the downpipe from the exhaust? - It's the flexible one covered in a wire mesh. These can deteriorate and allow a lot of emissions to escape and also stink the cabin out (Yes, I've had that too!!!

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