Thank you. from what you say, you have I think completed most of the initial steps that would be recommended in such a circumstance. Namely, to turn off all gas appliances in your property and check to see if the metre is still registering a reading which from what you say it is. This would usually indicate the possibility of a gas leak though from what you say, this is been ruled out by your gas engineer. If you are satisfied there is no gas leak then you need not concern yourself with this next step but if there is any concern in this regard, you should consider contacting the National Grid emergency line on 0800 111 999.
If a gas leak has been ruled out but the metre is still registering a reading even though all gas appliances are switched off, this would indicate a faulty meter and this should be reported to your supplier. Your supplier should offer to arrange to have the meter tested and if they do not make the offer, then you should request that it is tested. You cannot be charged for the test however it is important to note that gas meters cannot always be tested on site and may have to be removed to be sent to an independent lab for testing. Where this is the case, the supplier can make a charge for the costs of removal if no fault is subsequently found, there are in this case, that seems unlikely.
Under the supply code of practice the supplier must arrange for a test to be carried out following your above report within five working days or they must pay you £30 compensation and a further £30 compensation for the next five working days and so on.
Assuming a fault is found either by the engineer the supplier sends or by the independent laboratory which providing you have taken the above initial steps seems bound to be the case, then you will need to agree with the supplier and agreed reading which would typically be based upon your normal usage. Assuming you are able to agree such a reading, that may be end to the matter if you are satisfied but if not, you can escalate the complaint to the supplier which they must respond to within the maximum period of eight weeks and you can ultimately escalate the complaint to the energy ombudsman for independent review should you not be satisfied with the outcome of your complaint with the utility provider