If its the first time it has flowered perhaps its the wrong plant?

Other possibilities:
A volunteer seedling--courtesy of a bird--may be sprouting up through the 'Paul's Scarlet'.
The 'Paul's Scarlet' was grafted, and the understock is sprouting. See if the white-flowering branch has different leaves, and originates at the base of the bush.
At some point, a growing point mutated--called a bud sport--to disable the gene responsible for the red color. Bud sports aren't all that common, but white flowers are the commonest result of it. See if the white flowering branch starts up higher on the bush, and has the same leaves as the rest. A few branches with half and half, or striped, flowers may be present.