I would second (or third) the possibility that the temp shock might have been the last straw for the weakened fish. The fish are/were in a weakened state given the ammonia/nitrite that they have been exposed to for so long.
I don't know about you in the UK, but here in the US they make adapters that allow a garden hose to be hooked up to the kitchen sink. I use this with my tank, and I am able to closely match the temp for the water changes. I aim for about a 10 degree or less difference. I also aim for the lower side, as my fish prefer slightly cooler temps rather than higher. (The cooler water emulates a rainfall.) The bigger issue though is that if you are doing a massive 90% water change, and the temp isn't very closely matched, the temperature fluctuation will be far greater and a bigger issue. With weakened fish, it could be too much. Unfortunately, ammonia and nitrite can be silent killers - especially nitrite. There isn't always a symptom before it hits. Nitrite might show up as sluggishness, as the nitrite acts much like carbon monoxide does for humans. Ammonia gives a bit more symptoms with red gills or gasping at the surface at times. But that isn't always the case. The other thing about ammonia poisoning is that it continues to be an issue even after the problem has been dealt with. (It's like the after effects of smoking on people. Once the lungs are damaged to a certain extent, they cannot repair themselves.)
I feel terrible that just as your numbers were coming along the way they need to be that the fish are not dying on you. I can only imagine how frustrating this must be for you. You have worked so hard to get things right and now that they are much better, the long-term effects of the previous problem has come back to haunt you. Honestly, I don't know that this was avoidable. It might have been possible to delay it a few more months, maybe even another year if the water being warmer, but this was the eventuality. These fish weren't going to be capable to survive to the upper limits of their life span average. These fish could be compared to long-term heavy smokers. It was just a matter of time before it caught up to them. I'm sorry for your losses.