Just sign up QB, and connected my business account, and it shows a transaction of over £100 to be taken by 'Hartin Hardware' which I don't recognise. Please help.

Welcome to the QuickBooks family, @weichen818-gmail. Let me get the help you need with your bank transactions.

QuickBooks Online (QBO) is a read-only app. The data we receive in QuickBooks depends on the information sent to us by the financial institution. To investigate the £100 to be taken by Hartin Hardware, I recommend reviewing your bank statement and comparing it to the transactions downloaded on your QBO account. Then contact your bank so they can check it further on their end.

Also, once bank transactions download, you can now start categorising or matching them to straighten out your books. After that, you have the option to do your regular bank reconciliation. 

For future help, you may consider using bank rules. In this feature, transactions from your bank will automatically be categorised.

If you have additional questions while managing your bank transactions, let me know. I'll be around to provide further assistance. Have a nice day.