P115 CEEFAX 115 Sat 13 Feb 22:31/54       Britain's biggest nurses' union, the Royal College of Nursing, is to hold a secret ballot on whether to retain its no-strike policy. The ballot is to be held immediately and the results are expected bz mid-March, said RCN general secretar8 Mr Trevor Clay. He said he wanted to get the issue out of the way as quickly as possible so that the public and the media could concentrate on the real problem - the future of the health service. A two-thirds majority is necessary to overturn the no-strike policy, which has been rejected twice bjforj.