OK, so koo-power battery has arrived, seems to be OK to start with, charges, fits, all good.
Now, on with the show.
Here's the guts of the Makita
With 14v output, I reckon that's 4 duff cells or thereabouts, I have already found 3. Before I strip off everything, I was looking at the additional components. The bit on the side of the top two cells is a heat sensor, the one cemented on the top of the pack looks like a tantalum capacitor and appears to be wired across the output ("20K" is the printing on it), but why cement it in place? Anyhow that cell it is cemented to is OK and the two top ones are duff so it isn't that important as I shouldn't have to take it off.
More to follow, but it is looking interesting.