is the smell coming from the plant itself or the compost?

Compost - I think but I’m not sure.

Well its not the best time of year to do it but I would tip it out of the pot and wash off all the old compost. Wash out the pot thoroughly, and repot in a fresh coarse compost - even a cactus compost - you can buy it in small amounts.
Check the drainage - the plant should never be left really wet for more than the day you water it. If the amount of root is too small compared with the volume of compost and the compost is then left too wet you will begin to have problems such as rot or even a bit of a pong as you have seen! having the compost on the dry side will encourage new roots.
See what the others say - I have grown cacti for a long time but never had a Sanseveria. The above is what I would do with a cactus or succulent in that situation.Succulents are designed to live in semi dry conditions and need watering a lot less frequently than non succulent plants. That's why they have thick fleshy leaves.If they don't use a lot of water then when the leaves are full it will sit in the compost with the result you have seen.

that sounds like there is some rot in there. remove from pot and any that are firm repot in a free draining gritty compost. any that are soft/mushy discard. they don't like lots of water.

I don’t water it/them often but both are quite tall. Currently the smelly one which is bushier than the other is sitting outside. I will get some cactus compost though and hope it won’t have thousands of black flies in it! I’ve been battling with those wretched things for weeks now and still they keep coming. My Anthuriums are also outside in the shade because of the flies despite the sticky fly catchers.