Sorry John, i altered my post because I didn't want people thinking what the heck am I talking about, yes I have to take it to a regular garage who knows what he has to do, if I took it else where they fail it on emissions. I don't drive this car more than 10 miles each year, its being slowly restored as a classic. Currently undergoing some welding to its chassis.
(Although visual smoke testing is for pre 1975 cars I think, mine was first registered in 1987, so it does have to go through emissions testing but because it does not have a catalyst, it has a special category many garages do not cater for so they choose nearest category and I think what they choose is a category with a cat, and it fails on high co2 contents, but the garage I take it to has figured out exactly the right category it should be in, and last year its CO was 0.26% well under max permitted level of 3.5%, and HC (hydrocarbon) 851 well under max 1200ppm.
this year it is likely to pass again I hope as I have only driven it around 20 odd miles. But may be given some advisory on some rust patch near battery tray, that it may fail on. Will probably need welding at some stage.