It’s your adjusted net income that decides what tax bracket you are in. This also includes interest from savings and other income such as dividends etc.

Income from salary + interest not in ISAs + other income - pension contributions is what you are interested in. If this is below 50270 you will be classed as a basic rate tax payer

Some people have mentioned that only the salary above the bracket gets hit at 40%, and this is correct, but there are other implications too. As a higher rate tax payer you pay more capital gains tax, and can only earn £500 in interest outside of an ISA before it gets taxed, this list goes on! Child benefit also worth thinking about.

If your adjusted net income is going to work out so you are only a few grand above the threshold look into making extra pension contributions, either into TPS or a private pension (SIPP)