Few things to discuss here.
First, as you stated, I didn’t use the word guarantee because I think it’s a bit of a meaningless phrase, but I still think it’s very, very likely that the NI hike does push up on inflation over the next few months. The inability to guarantee that something occurs doesn’t reduce the prospect of it happening or make its impact any less severe.
The more substantive issue relates to the proportion of businesses planning a price rise in relation to the NI hike and how this is likely to lift inflation.
The fact that only just over half of the businesses sampled are planning to raise prices, and not 80 or 90%, doesn’t really reduce the risk of a spike in inflation.
If you look at the underlying price data sampled for the CPI by the ONS each month, it really isn’t uncommon to find that half or even a majority of prices remain unchanged over the month. The key issue is obviously how the price change observed over the month relates to the price change in the corresponding month of the previous year. Given that the NI hike is a new factor and not related to the normal seasonal pattern of price change, I would think it very likely that monthly price change will be unusually strong for a period and therefore boost inflation. Even if the NI hike only causes businesses to bring forward planned price increases, I would still expect price growth to remain stronger than it otherwise would be on a year to date basis, and therefore boost inflation.
Also, describing the DMP survey as a crowdsourced guess is a bit OTT. It does provide good information and its message in relation to the NI hike has been backed up by other surveys.