This happens to me at a petrol station I sometimes go to.....I normally notice it come about 3 days later.
Hopefully it won't come out ur bank!

You could try ring your bank and they'll tell you if there's anything pending for £200.
Another thing to watch out for is that they only bank once a week (the slips have to be sent off by the bank & then get processed) So if their banking day was saturday and they'd gone to the bank before you'd shopped then they wouldn't take the slip for your amt until this saturday.....then it would get processed next week. (hope that makes sense)
I bought an MP3 player (when they first came out) from dixons The Boy on the till was really chatty & not concentrating. I was paying by chq and he didn't write my card number on the back, printed the chq in the machine & never gave it back for me to sign.............that was almost 3 years ago......I've still never paid for it!