Agree with both of these. discoinferno is referring to the written statement of principal terms and conditions which the employer is legally required to supply within the first 8 weeks of employment, and which should indicate, amongst lots of other things, the notice obligations of employee and employer. The problem is that there is no penalty on an employer who fails to carry out his statutory duty, so there is widespread failure to comply...
...except that, in similar situations where worker and employer are in dispute over a particular 'right', I have argued successfully in Employment Tribunals that the worker's interpretation should be followed because it is the employer's fault that there is no written statement covering the issue. On that argument, dOgsbOdy could go for the one week interpretation suggested by bednobs, provided s/he is paid weekly.