MWB operates serviced offices in central London. Rock entered a contractual licence with MWB to occupy office space in Marble Arch and had accumulated licence fees in arrears. The original licence agreement contained a ‘No Oral Modification’ clause that said: 'All variations to this licence must be agreed, set out in writing and signed on behalf of both parties before they take effect'. After 6 months, Rock director re-negotiated to extend payment period over phone call and MWB credit controller agreed his proposal. Is this agreement considered as an effective variation to the original licence agreement?