The extra payment is to help compensate Mr Henry for the loss of benefits from the executive share scheme operated by his previous employer, Halliburton.
The revelation comes amid growing concern over boardroom pay and perks. Mr Henry defended the upfront payment, of which £40,000 went in tax. He said that he invested the remainder in National Power shares, demonstrating his commitment to the company. He also said that the row over pay in the former nationalised utilities is "largely overdone".
Mr Henry succeeds John Baker, whose pay last year was £304,500. Mr Baker is now part-time chairman on a salary of £180,000. Mr Henry, whose basic pay is £280,000 with a bonus of up to 40 per cent, will also benefit from an executive share scheme based on performance over three years. He has not yet been granted share options, which are decided by the remuneration committee in October.
In his first public statement since joining the company, Mr Henry said that National Power would not be forced into selling of power plants, as demanded by the regulator Stephen Littlechild unless the benefit to shareholders was clear. "We would rather not do it. We are not as afraid of the Monopolies and Mergers Commission as Professor Littlechild may think we are."
He added, however, that the company is evaluating two offers and is still considering a demerger within the group. Professor Littlechild wants National Power to sell about 4,000 megawatts of capacity, which would affect 700 to 800 jobs.
Mr Henry said Offer's demands are at odds with Government plans to merge and privatise Nuclear Electric and Scottish Nuclear.
He also warned of the threat to National Power's ambitious overseas expansion plans. "The more we get split up, the less chance I have of walking through doors overseas."