I can overcome this by putting a ‘$’ in front of the parts of the address I don't want to change.
In this case I don't want the '2' in the address of cell 'B2' to change, so I change the formula to:
Now when I drag down the formula in 'D5' becomes:
=2*C5 + B$2
You can add the ‘$’ to the row or column or both. I would normally have added it to both:
=2*C4 + $B$2
Then I could have copied the formula anywhere without worrying about either the letter or number.