thank you for replying to my query.
Well I decided that it would be good to confirm whether there was an issue with T1 secondary windings in respect of them being either O/C or H/R. So I removed each leg that feeds pins 7 and 9 of IC4, this revealed that the centre tap connected at the junction was 2.5V when power was applied.
T1 secondary is OK.
C1 is OK and not S/C, so I must assume that IC4 is at fault when connected to T1 secondary via pins 7 and 9 and that maybe the method by which it appears to have failed is the reason for it dragging down the 2.5V derived at the junction of R1 and R2, hence this potential cannot propagate through to bias IC5 tending toward IC8.
On inspection the pins on IC4 look OK in terms of soldering and being free of any S/Cs.
Maybe the next stage is to perform surgery and remove the current IC4 with a new one - fortunately I have one in stock.
Thanks again, I hope you are keeping well and safe at this difficult time.