I take it from your way of speaking that you are talking about a UK Deed of Change of Name? Wikipedia gives an interesting article about it...http://en.wikipedia.org/wiki/Deed_of_change_of_name

It looks as if your best bet is to check the London Gazette for the current name as it appears to have been a requirement to make some sort of notice of change. I don't know, however, if they were required to put up both names or just one or the other. At least it should get you close to a date for searching the records of name change listed in the High Court. Those records should be at Her Majesty's Court Service: http://www.hmcourts-service.gov.uk/

Hope this helps!