Call to undefined method TCPDF::SetDefaultMonospacedFont()

Call to undefined method TCPDF::SetDefaultMonospacedFont()

I got this earlier today when clicking the Cart Enquiry button.
I've looked up online what it might mean and get references to fonts but not much else. 
The data actually arrives to me as admin but in a nasty looking page of stuff - not very elegant.
And when the customer goes back to the page the cart contents have disappeared. It doesn't send a copy to the customer either - although I thought I had it set up that way/
Any suggestions?