You're effectively gambling the interest that you would receive in a savings account. You might win enough to match or beat the interest, or you might win little or nothing.
If you're starting from scratch and saving £400 pm, you'll have saved around £5k in a year's time, with the average balance for the year being £2.5k. The best interest rate you can get is just over 1%, so £25 interest would be earned in a savings account.
With PBs and that amount, there's a good chance that you will win nothing. But if you win one £25 prize in a year, you're in the same position as you would be with a savings account, any more than that and you're ahead.
So you need to decide whether you're happy to gamble the guaranteed £25 against a maybe 20-30% chance that you'll match that with PBs and a much smaller chance that you'll win a bigger prize.
I have just over £14k of PBs that I've built up over the last year or so. In 2019, I won 5 x £25 prizes, which worked out at a payout rate of just over 2%. This year, I've only won one x £25 prize so far, so am currently losing against the interest I would have got in a top savings account.
Incidentally, if you can stretch to £500 pm to save, NS&I also do an 'income bond' which pays 1.15% interest. It's effectively an instant access savings account, but the minimum pay in or out is £500.