[Mr Gazza]

Your comment on 10V when the starter is pressed does not ring any particular alarm bells.
It's usual to get a significant voltage drop when the starter is cranking.
To get a better picture, put your meter across the battery terminal's. Expect to see 12 to 13 volts with the ignition off. Read the voltage when cranking to measure the voltage drop. When the engine fires and runs, the voltage should recover almost immediately to the former and rise to about 14.5 volts as the charging current comes through.
Excessive voltage drop is caused by a dying battery losing it's Cold Cranking Amp ability (CCA), and/or high resistance in the starter cables. Nice bright connections on the heavy leads throughout the starter circuit help to reduce the voltage drop... Don't overlook the heavy Earth lead connected to the engine (or frame on earlier models).

Do be careful with the starter terminal stud. Make sure that it doesn't turn when you undo the nut, as this will mangle the connections inside the starter!

I think it's unlikely that your problem is in this circuit although it's a good regular service item anyway.