I've treid to find the template for these and have raised a support ticked with your team and given this response
Thanks for getting in touch.
I have tried to change the Reminder letter template in our system but it seems at this moment this feature is not something that we are able to offer.
This does seem like a good idea though so it would be great if you could add your suggestion to our KashFlow Canvass website where KashFlow clients can share ideas directly with our Product team.
Sorry I couldn't be more help with this but feel free to let me know if you have any other questions.