When Vaillant came about in the 90s, they were excellent but if buying one now, you need a Bosch /Worcester.
Knowing about the cost of combi repairs, i keep the boiler i fitted back in the 1980s in my house. cast iron heat exchanger, no electronics, just a thermocouple and a thermostat. Works very well in a house with quite a large floor area and on 3 floors.
Slow to heat a cylinder of water is the downside but i can boost that with an immersion heater that just heats the upper section in 15 minutes for a shower.
The electronic boards in most combis will cost over 150 pounds and they do fail quite a lot on the early models