The legislation is at:
Taxes Management Act 1970
Taxation of the interest is described in link below and in particular 'If an individual is unable to withdraw or have access to the interest when it is credited to their account, or has a specific product such as a bond, the interest will not arise and therefore they will not be taxable until they have access to the interest.' if this applies to you then the interest should be declared when you can access it(maturity).
SAIM2400 - Interest: taxation of interest: the tax charge