Ah, right, understood. To be honest, it shouldn’t make any difference whether you go en masse or drip-feed. The random nature of the kit that selects the winning numbers (‘ERNIE’ as it’s commonly called) means that every individual bond is as likely or unlikely to win in any given month as any other.
I just checked mine, and of the 5 winners this month, 2 of them start with the very same sequence 383LH43…. with the last 4 digits being only about 2500 apart. So those were clearly bought at the same time as each other, i.e. as part of the same original transaction, maybe one occasion when I put £5K in. The other 3 winners of £25 apiece are separate sequences entirely, so must have been bought at different times.
If you’ve got the cash to put in, might as well do it en masse, and then effectively it’s all in the next month’s draw, rather than being spread out over several months.