Yes, providing you don’t exceed the maximum annual allowance you can add to your ISA at any time during the tax year. If you are not paying in every month by direct debit, the minimum additional investment is £100.
In future tax years you can top up your existing Sheffield Mutual policy or you may, if you prefer, open a new policy for each tax year. Having separate policies may provide you with greater flexibility in the future. For example, if you have separate policies for each tax year and you want to start withdrawing funds, you could perhaps surrender one of the older tax-year policies to crystalise the annual bonuses and receive any possible final bonus.