Previous owner had replaced a stack of resistors in the LH channel. All symmetrical so got the multimeter out... every replaced resistor was an incorrect value compared to the original equivalents on the RH channel. Quite a few PCB tracks burnt out too, Have lots of caps and resistors lying around, acquired some BC139 and BC140 and have steadily undone all the bad work - haven't powered it up but if the output devices are gone I'll just junk it... just thought I'd try to give it the kiss of life so as long as it didn't cost me much.
Anyhow, just two resistors left - R 47 and R 48 - located just to the left of the LHS green J119 Mosfet (?) on the heatsink. Both are 4.7k ohms though one's open circuit. Unbelievably neither are actually connected to the PCB at either end! Given the other resistor value errors I'm dubious about the accuracy of the values selected for these hence the original question.