All UK market Dolomites changed from single line to tandem on a date around the end of 1978, the change being prompted by govt legislation. The ACTUAL date is not known, but it coincides, as far as I can tell from extensive research, with the introduction of the so called "series IV" Sprint, commission number VA 30000>. Some export market cars for certain countries had tandem brakes as early as 1972, these cars had a different master cylinder to the later UK tandem cars.
But the cars have been around long enough that quite a few earlier cars have been retrofitted with dual circuit brakes, including my own 73 Toledo and 78 (May) Sprint Auto. Both of which came into my possession as single line cars. Which is enough to confuse any one!
When I first got a sprint in 1992 I could not believe it had a single line brake system. My TR5 that I had owned a decade earlier had a dual and it showed the effects of a company ruled by bean counters rather than engineers.