Thanks for your speedy response @RaymondJayO , much appreciated. 

Is there any way for me to get around the issue that an invoice can't be marked as paid if it wasn't sent through QB? Because if so, I can live with downloading the invoice PDF and emailing it separately to my customer.

Alternatively, I could probably live with the customer having to make all the clicks if it weren't for the sub-heading, which you've mentioned is a known issue - do you know if that's something that will be resolved? I imagine the vast majority of my customers check their email on their phone and this really does look unprofessional. 

Thanks for your support - I find the banking/expense functionality brilliant, but the invoicing part is equally important to me and the experience for my customers just isn't what I had hoped for.