I think I've finally got there ! Making as close to 25 ohms as I could, two 12 ohm resistors in series, so tried 24 ohms for R7.
Voltage across R7 was 1.186 Volts unloaded, and 1.9 Volts with a 50 watt load on the final output.
The voltage across the 0.1 ohm 10 watt ceramic resistor was 0.469 with the same 50 Watts in the output. So I guess this value of AK's 25 ohms is ok. I'm not sure what was going on with the 100 ohm resistor value for R7 giving an output of 17 Volts. I am at a loss there !
Voltage across R7 was 1.186 Volts unloaded, and 1.9 Volts with a 50 watt load on the final output.
The voltage across the 0.1 ohm 10 watt ceramic resistor was 0.469 with the same 50 Watts in the output. So I guess this value of AK's 25 ohms is ok. I'm not sure what was going on with the 100 ohm resistor value for R7 giving an output of 17 Volts. I am at a loss there !