Gas reading lower than previous

I am being repeatedly asked to submit gas readings as the number I Am Submitting is lower than the previous one apparently. I am not sire if there has been a mistake in the past, but I am submitting with photographic evidence. I have had an extremely worrying letter saying I will owe £1500 - I have requested a call back and have had no answer. This is making me extremely anxious as you can imagine. I have looked everywhere for a telephone number with no luck except for emergency gas leak etc. Some advice please on how to resolve this. Thanks

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Thank you for your help