The FTSE100 index closed at 1630hrs UK time yesterday at 5842.66.
However, traders are continuously offered quotes in the index value by their brokers even while the London Stock Exchange is closed. It follows that although the underlying FTSE100 index was last calculated at 1630 yesterday at 5842.66, that was not the time that your broker "closed" their FTSE100 quotes for Thursday. Drop down to the 30-minute charts - their closing price is probably from their closing time of 2200hrs.
I think there's still something that needs to be investigated here.
My spreadbetting live account firm quotes the FTSE100 closing at 5841.98 at 1630 yesterday. That is their bid price, and is only 0.68pts out from the underlying index value so no worries there. The broker's ask price quoted is higher so the spread would explain the tiny variation.
However, your broker's quoted 1630 price is 9.29pts out - too much. That means they either operate an expensively wide spread or they're using a slightly different time for their closing value.