Gas leak - help!

Firstly, thank you so much for all the helpful responses!!
Secondly- Sorry, I frantically wrote this message and may have been a bit misunderstood.
I'm 27 and live in America, my mom is 49 and lives with my stepdad (who is a bit useless with this sort of stuff) and 3 teenage siblings. She's not an old lady living alone.
She is in charge of the bills and has been aware of the smell in the cupboard for a long time but thought it just smelled old and musty. I went in the cupboard and thought it smelt like gas and my stepdad said he had thought that too.
This afternoon my Nan and grandad visited and both smelt the cupboard and thought it smelt of gas too. We all had a bit/lot of a panic and an argument about the next step to take regarding the landlord and I posted on here.
I think she shouldn't live here because she will always run into these problems with the shady landlord! She's taken to paying for repairs herself as it's such a grind trying to get the landlord over for anything! When the water first went he came and fiddled with something and it worked again, it stopped working shortly afterwards and she fiddled with the same thing and it worked again, it stopped working again and she couldn't get it working again, the landlord came out and got the water to barely lukewarm and said 'there you go, that will heat up' it never did. By this point she was fed up with him messing her around and they all just coped with cold water. There was recently a water leak, the landlord eventually came with a plumber and fixed it. My mom told the plumber about the hot water and he agreed to fix it. Long story short, the landlord told my mom to pay for the work and take it off her rent at the end of the month!!! Plumber was a no show, landlord is now in India for 2 months and said he would sort out the hot water when he is back in the country.
The reason she thinks 'leave them alone they'll leave us alone' is because she has 2 dogs and most landlords won't allow pets in their home so her choice of house to move into is limited. The location is good as it's close to work and the property she was due to move into originally was literally pulled from under her feet 2 weeks before she was due to move in making my family effectively homeless. Luckily my Nan and grandad have a big house and we're able to take my family in for 3 months. My mom was desperate to move out and so the first house that came along that ticked all the boxes she grabbed with both hands despite the state it was in... (rented to a man who illegally sublet to 3 young lads who trashed the place. I mean TRASHED- my mom then cleaned it herself and forked out to decorate rooms and get it to a liveable state)
She allows entry, doesn't have an issue with that. Nobody came by for almost a year, she thinks, to take a reading . She had been paying £150 per month which is in line with other similar properties she's lived in over the last 10 years. A reading was taken and she was told that she owes £1500

So, I remembered (and reminded my stepdad- don't know how they get through the day without me ��) that one of his old friends is a gas man. He text him and he's coming round tomorrow. He said in the meantime just shut off the gas (well like I said, the heating is never bloody on, we are out this evening so don't need to cook and they're used to cold water so not an issue!) but I'm thinking that we should have called national grid. I asked my stepdad to ask his friend if we should call NG and he said no, just shut it off and he'll come round tomorrow.

Like I say we don't know for sure that it's a gas leak but I'd say the hefty bill and smell off gas maybe adds up?

Issue is we're dealing with obviously shady landlords here...
the property was advertised through an agent initially but now is managed privately by the landlords sister. 1 months rent deposit was paid which my mom believes is held in a scheme..
Anything she goes to them for they drag their feet then try to fix themselves with an absolute botched job! (Shower broke and took 2 months to fix, oven took 6 weeks - side note, they tried to silicone seal the glass back into the front of the oven when it kept falling out! Then removed it from the house when that didn't work and brought it back 6 weeks later!)
All they care about is getting money each month and keeping costs down on anything that needs doing.
Gas inspection has never been done, nothing to do with my mom refusing entry.

She doesn't want to rock the boat because she doesn't want the struggle of finding somewhere else to live if they feel that my mom gives them too much grief.

Where do you think we stand when it comes to the bill if it's proven to be due to a gas leak?
We haven't contacted them yet as I fully expect them to try to send over one of their friends to do an inspection or botch something up or try to pull the wool over their eyes for their benefit.

Sorry that's been a long one! Hope I've answered all questions. Difficult to explain the full situation. Thanks to everyone for taking the time to read and help. Really appreciate it.

Kelly