I refer the hon. Member to the answer given by my right hon. Friend, the Secretary of State for Defence on 3 December 2009, Official Report, column 911W. As for any modifications planned for this system, it is Ministry of Defence policy not to comment on details of UK warhead design as to do so would, or would be likely to, prejudice national security and defence in the UK.
The overall cost to the UK of procuring the Mk4A component was an element of the estimated future costs of the Atomic Weapons Establishment as set out in chapter five of the White Paper ‘The Future of the United Kingdom's Nuclear Deterrent’ (Cmd 6994), published in December 2006.
I am withholding further details on the costs of the Mk4A programme as their release would, or would be likely to, prejudice national security and defence in the UK.