The last two posts seem to imply a different situation, the first an external auto bypass and the second the internal boiler bypass.
I meant the boiler's internal by-pass. There shouldn't be a need to install another (unless there is another pump).
Anyway, even with a return temperature of 55°C, the boiler would only just
begin to be condensing and the savings are three parts of naff all. Condensing improves all the way down to 30°C return. Maximum efficiency is stated with a flow of 40°C and a return of 30°C (UFH territory).
The flow through the radiators needs to match the design flow through the boiler (even if the temperature drop across the radiators has to be reduced). That way, the by-pass can stay closed and significant condensing can occur at low return temperatures as the radiators warm up from cold. From the manual charts: With the by-pass at the factory setting of 0.25 bar, you'll need at least 1200 litres/hour flow through the radiators to keep it closed. This corresponds to a MAXIMUM achievable temperature rise across the heat exchanger of about 15°C.
If you crank up the by-pass to the maximum 0.35 bar, you'll only need 900 litres/hour flow to keep it closed. Even 900 litres/hour at 20°C rise would be 20 kW. So, if your radiators total 10 kW, you should set the temperature drop to less than 10°C or the by-pass will open and condensing will stop. Basically for this boiler,
work out the total radiator output in kW and set the temperature drop to the same number of °C and crank up the by-pass to maximum. Turn down the flow temperature in mild weather. Even after all that, when the TRVs close, the by-pass will open and condensing will stop (or at least reduce significantly).
Unfortunately*, the boiler is conservatively designed to protect the heat exchanger against excessive temperature rises without any sophisticated controls, even with no external flow. Some boilers can modulate their pumps to maintain efficiency and others modulate output to protect their heat exchanger, but not this one.
*This might make the boiler more reliable and reliability is much more important than efficiency.
If the radiator output is much less than (19kW) boiler output (it usually is with combination boilers), you might want to de-rate the boiler - see manual "5.11 Adjusting the central heating output (range rating)". With all the TRVs open, I'd reduce the power so that the boiler takes at least 15 minutes to reach set temperature from cold. At least that way, the boiler will spend more time condensing as it warms up. As load reduces, the room thermostat might turn the heating off quickly before the boiler stops condensing.
I predict that the next generation of high-efficiency boilers will use nearly stoichiometric gas/air mix and have a catalyst to control carbon monoxide. They would run with CO2 levels close to 15% and would start condensing at much higher temperatures. After that, they will have to start conserving entropy rather than just enthalpy and run with efficiencies of several hundred percent as gas powered heat pumps.
