I can imagine that would be concerning, there are a few things you can check.
Firstly, check the previous invoices on the second page to see if they are Actual or Estimated, if they are estimated, that would indicate that we have not been able to retrieve a meter reading and had to guess, so this bill would be any uncharged usage over that period.
The other check I would do would be to check your gas meter reading on the meter, compared to the last bill, see how much gas has been used since you got the bill to see if you are still using high amounts.
You can also get some more information about energy reduction here which also shows the difference between Summer and Winter.
Please touch base with our dedicated Messaging Team if you need some help investigating this.