Return today's value - data source pushes daily cumulative total!

Hi all

I have a tricky situation that has stumped me.

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'Campaign Stats' = data pushed from DotDigital email marketing platform, appending a row for each day of activity for each campaign. The final column is a 'Lookup' for Vlookups. Please don't manipulate the data in this sheet
'DD Daily' = This is the sheet we're working on
'DD Running' = This simply shows us the data in 'Campaign Stats' resorted to match the order in 'DD Daily'

The objective - As you can see, the data from DotDigital is provided as a cumulative campaign total for each day (note that the first day for each campaign will be a much bigger increment than following days, that is expected). What we actually want is the total for each individual day's activity. You can see in columns F to H that I've got tried If but realised that this will only substract the proceeding day - what we actually want to do is substract the earliest day if the row is not itself the earlier day for that campaign. I suspect MIN might be part of the answer, but I can't figure out how.

A solution would be fantastic!!!!