Would an extra 10mg diazepam on top of what I'm already prescribed to take - how would it affect me?

Hi, hope!
It would still be a relatively low dose but it could cause you quite a bit of sedation you're not used to.
If you feel a need to increase your dose you should it report it to your doctor for professional advice.
Benzodiazepines should only be used short term and at the prescribed dose as they are addictive. Also, you won't be able to get a refill if you use up the medication before it's time to refill it.