5 March 1960
On 5 March 1960, the South African and British governments were in open conflict over the interpretation of the Simonstown Agreement. The South African government had accused the British of being in contravention of the agreement by suspending supply of arms. The British Foreign Secretary Michael Stewart gave a reply to the South African communication, stating that Britain was honouring the agreement by continuing the arms embargo. The response was presented to Hendrick Lutting, the South African Ambassador during a meeting which was requested by Stewart. The content of Stewart's reply was kept a secret by authorities, but observers believed that the British stance showed that they were not bound to supply arms indefinitely. The British claim that all the arms that South Africa requested and  listed when the agreement was drawn up on 30 June 1955, have been provided and that there was no further contractual obligation. Seven years after the open conflict of 1960 between the two countries, the head of South African Defence Force, Commandant General Hiemstra announced the successful conclusion of Anglo-South African negotiations on aspects of the Simonstown naval agreement for the defence of sea routes around Southern Africa.
References

The Cape Times(1960), 'British -S.A. Conflict on Arms Embargo' from The Cape Times, 6 March 1960 (Available at National Library of South Africa, Cape Town Campus) pg 1.|

Jstor (1971), 'Britain's Obligations under the Simonstown Agreements' from Jstor, [online] Available at www.jstor.org  [Accessed: 13 March 2014]