Philip Latham, the Lib Dems' Stockton North candidate, said Mr Cook's decision meant that Stockton North was now a marginal constituency.

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Official figures released in a parliamentary written answer say more than half the official visits made by David Jones, Stephen Crabb and Baroness Randerson have been to this key marginal constituency.

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Although they are in coalition nationally, Mr Clegg said there were "very clear" differences between the policies of the Lib Dems - who control Eastleigh council - and the Tories in the marginal constituency.

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