Could it not be possible that the injector cleaner (im asusming something like redex) was burning off crap and producing some shite, especially if it was fairly concentrated in the tank. I remember pouring it down my carb back in the day and producing some right black shit as it burnt off what i would asusme were carbon deposits etc.

Someone else mentioned a mot fail, personally as an owner of a older car there is no way id let a car go in for mot with things like broken springs etc or even go for its mot without me being present whilst its being done. The cost of 4 cheap car ramps is bugger all and lets you get a good view of the underside and check for corrosion/exhaust holes/bust springs etc before it goes to be tested. Just my opinion tho