Do you have to pay in every month or can you put the full amount in the day before the tax year ends and still get the full benefit.

Well it's the interest that is tax-free isn't it?

So you can put the money in any time and still benefit, but if it just one day before the end of the tax year then you'll only be getting one days interest. But it will be tax free. 😀

But, you can then put in your allowance again in the next tax year. The benefit's that it's tax free but the restriction is per tax year. But since we're pretty much halfway through the tax year that's not really a concern just now.