I'm quite surprised you would get a letter like that from IG when you are in fact trading within the frame work they themselves provide.
I'm guessing if you have short term 'long positions' where its possible a day trader loses track of time and on occasions holds at market close IG would still expect overnight funding payment and would not deter you from further trading.
As a side note, the front and next futures have converged to make short position arbitrage type trades less appealing, but in the near term as the June contract nears expiry its possible that they diverge, maybe to levels similar to late April. Interesting times!