Not helpful but my mate recently crashed his Boardman bike and has bent the left Tiagra lever.
If you can't source a replacement right lever and end up buying a new pair, would you be willing to sell/donate the left lever?
He's still considering his options to get his bike fixed - but as his waiting for his knee to heal he's not is too much of a hurry.
EDIT: Alternatively, I'll ask if he wants to sell/donate his working right and buy a new pair. Would the levers on a Boardman Road Comp bike be interchangeable with yours?