I've been thinking about this and how it all makes sense in hindsight. This would be my reconstruction, and hopefully people will tell me if they think it is wrong:
The OP had a sealed system (primary water) at 1 bar (estimated) pressure. That's the heating system. There was also a mains pressure cylinder. The two systems should not be able to mix at the coil, but the coil was starting to leak.
Presumably the mains pressure is not that high (or there is a pressure reducing valve) so the pressure in the cylinder was not much greater than in the sealed heating system so some kind of rough equilibrium developed without the heating system PRV having to open. Some mixing of the two water systems occurred as pressures fluctuated, but this was minimal as the 2 systems had roughly equal pressures. Enough to lead to active corrosion and brown water in the heating system (rather than a small amount of black sediment), but not enough for the OP to notice a problem.
Then the mains failed. At this point, the heating system pressure was much higher than the pressure in the cylinder and a large quantity of heating system water had to run into the cylinder to restore equal pressures. This may have enlarged the existing leak and noticeably contaminated the cylinder water.
If the OP does not intend to replace his boiler immediately, I would recommend that when he has the cylinder changed he makes some effort to improve the (probably very poor) quality of the primary water. At least several fills and draindowns or a mains flush, and then refill with quality inhibitor. It is surprising how long a boiler can run on borrowed time if looked after, if you're lucky!
