The channel secured a 51% share of viewing between 8.30pm and 10pm, with the audience peak of 13.2m coming when the result of the night's vote was announced.
Saxophonist Julian Smith and singing grandfather/granddaughter duo Two Grand went through to the final.
BBC One trailed far behind with just 2.5m viewers and a 10% share for 9-10pm documentary 'Tourettes: I Swear I Can't Help It'.
Five's 'The Mentalist' was next best in the same slot, managing an audience of 1.5m and a share of 6%.
BBC Two garnered 1.2m and a 5% share with 'Keep It In The Family', showing people who were unwilling to take over their family's business giving it a try.
Channel 4's Cutting Edge documentary 'The Building Inspector Is Coming' was seen by 1.1m, a 5% share.
On E4 the last ever episode of US hospital drama 'ER' was watched by 1.05m at 9-10.45pm.
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