Yes, SP = setpoint, 70C seems OK, the return will get very cool when all the TRVs close in, theoretically, if no permanent by pass then the flow can go to practically zero, when I installed my Wilo Pico two years ago I took a good few readings with the circ pump on but (20 kw oil fired) boiler off and with just any one rad only full open I was getting a minimum of 5/6 LPM and with 2 or more full open ~ 7/10 LPM. so I would suggest making a note of say 3 rad TRV settings and then opening them fully for a few minutes and monitor the watts/flowrate. The boiler seems to be behaving OK though even with that small flowrate, your main problem IMO is probably a partial blockage in that Tower, I have a combined Vent&cold feed in my system which I consider to be the very best arrangement in a open vented system, my system is spotlessly clean with some of the original rads after 40+ years with only the odd drop of inhibitor added.
From your boiler spec, here are the recommended flow rates.
"Systems with Two Port Valves Where a pair of two port zone valves are used, the total length of the by-pass circuit taken from the boiler connections should be greater than 4 metres of 22 mm pipe. The bypass should be capable of maintaining a minimum flow through the boiler of 4.5 litres/min (1 gal/min). Systems with Thermostatic Radiator Valves (TRV’s) Where mechanically operated thermostatic control valves are used, the total length of the by-pass circuit taken from the boiler connections should be greater than 2 metres of 22 mm pipe. The bypass should be capable of maintaining a minimum flow through the boiler of 9.0 litres/min (2 gal/min)."
From your boiler spec, here are the recommended flow rates.
"Systems with Two Port Valves Where a pair of two port zone valves are used, the total length of the by-pass circuit taken from the boiler connections should be greater than 4 metres of 22 mm pipe. The bypass should be capable of maintaining a minimum flow through the boiler of 4.5 litres/min (1 gal/min). Systems with Thermostatic Radiator Valves (TRV’s) Where mechanically operated thermostatic control valves are used, the total length of the by-pass circuit taken from the boiler connections should be greater than 2 metres of 22 mm pipe. The bypass should be capable of maintaining a minimum flow through the boiler of 9.0 litres/min (2 gal/min)."
