Retropin,

This has been great - thanks a million.

Testing IC15 (8 as GND)

3  3.64
2  1.63
7  3.59
6  3.56
14 1.55
15 4.2
9  4.26
10 4.23

IC 16  (using 1 as GND - 24 pin chip)

3  3.21
2  2.68
7  3.16
6  2.85
14 3.57
15 1.41
9 2.57
10 2.32

Some of these are as you suggested - others are not

IC17 - you mentioned pin 7 as GND, but using circuit diagram it says pin 12 is GND (again 24 pin chip)

Using pin 7 as GND

1  0.24
2  0.2
5  0.6
6  0.16
13 0.66
12 2.26
9  0.47
8  0.14

IC18 (Big PIA Display Output Chip - circuit diagram says 1 is GND?)

using 7 as GND

1  -0.03
2  2.57
5  2.37
6 0.00
13 0.33
12 0.34
9  0.00
8  0.00

I await your instruction.....