Modifying an existing 3-cell BMS circuit to disconnect the pack at high temperature
You could just put your switch in series with R22.
The pulldown R16 will then turn off Q10 with the same result.

My question is do I need a capacitor in parallel with R8 to avoid false triggering i.e. any noise picked up by the cable from the thermal switch to the board?

A capacitor seems unlikely to be harmful.
Is the switch &quot;snap action&quot; (presumably yes) or like a thermistor?

Yes its a snap action switch once open at 40 degrees it shouldn't close again until it has cooled several degrees. I do however want to add some temperature hysteresis to prevent false triggering.
The idea is that normally JP1 (a NC thermal switch placed close to the pack) keeps Q8 turned off. When the temperature of the pack rises above 40 degrees the base of Q8 is pulled low relative to its emitter. This in turn will turn on Q1 which will turn off Q10 by pulling its gate to ground and disconnect the pack.

With the circuit as you've shown it's hard to see how you can add temperature hysteresis with an on/off switch. The worst you may get is a brief period of 'bounce' and if you don't find a few very short on/off cycles that seems harmless. If you want to eliminate even bounce then maybe a hysteresis resistor from Q1 collector so once it starts switching it is committed. This also gets 'feedback' from anything else that triggers Q1 - probably not a problem but you'd need to work through what happens 'just in case'.