although when I was over in Dusseldorf the other month I did take cash out the wall, it may well be this!
It will almost certainly be this.
In general, you're correct. As others have said, if you pay off the balance in full each month you won't pay any interest.
The exception to the rule is "cash advances". Cash advances aren't just when you take money from an ATM but can be when you buy Traveller's Cheques and foreign currency. Why you may not have "gambled", buying a lottery ticket would also incur fees.
Another reason why the interest may seem dispropriate to how much you took out in cash is that they can sometimes be charged at a higher rate of interest.
At the same time read up on what counts as "cash advances" and avoid making transactions like that in the future. You've done better than some people by catching it so soon and keeping on top of it!
If I am honest I track all expenses and purchases on a spreadsheet and go off that instead of the statement
That's fine, it means you'll pay off items before they're even due and you will likely avoid all interest, but you can also set up a direct debit for the "full amount" on the statement and (at least with my cards) it will only take the difference required to pay off the full amount if you have paid off some of it earlier in the month. However, you might want to double check that is the case with your card.
Also, it's easy it just say "set up a direct debit to pay in full at the end of each statement" but I know one reason I like to pay things off ASAP is that it means I don't accidentally spend the money before it's due a month later.