According to section 86 of the Companies Act 2006, a company must always have a registered office to which communications and notices can be addressed. Service of a document is effective if it is sent to the company's registered office as per section 1139 of the Companies Act 2006. When a company changes its registered office, it must notify the Registrar of Companies within 14 days of the change. The change takes effect when the notice is registered by the Registrar. However, for a period of 14 days beginning from the date the new address is registered, documents can still be validly served at the old address, as per section 87 of the Companies Act 2006. Given that the new address was registered seven days ago, the claimant can serve the letter of claim at either the old or the new registered address. 

Key Point: When a company changes its registered office, there is a 14-day period during which documents can be validly served at either the old or the new registered address, ensuring seamless communication during the transition.