As already noted, that's where the dishwasher salt goes, to soften the water.

Sluice it out as best you can with fresh clean water, so at least the water in there is 'new'. You may need to resort to a sponge & bowl/bucket to stop the bottom of the dishwasher flooding while you're working.  
Don't use bleach or other harsh chemicals - the resin beads used by a water softener can be damaged by bleach. Some beads can stand it, some cant - & we don't know which type are in there.   
Also, avoid bleach on anything made of stainless steel - [Does Bleach Affect Stainless Steel? Our Research Found This][1] - you may get away with a small amount, once or twice, but it can make even stainless steel rust. It can also stain the surface - which is not so much of an issue cosmetically inside a dishwasher.

Refill with dedicated dishwasher salt, not regular kitchen salt. This will make a lot of the water spill back over into the bottom of the dishwasher - this is fine. When full, wipe the screw thread before screwing the lid back on, to save the thread from becoming clogged and preventing the lid from sealing properly. Sponge out any spilled salt, or immediately run the machine on a short cycle, empty.

Buy a dedicated dishwasher cleaning product - this covers products specific to the UK market, so you may not find them all everywhere [Best Dishwasher Cleaners (2023 UK)][2]. It also includes a cleaning & deep cleaning guide at the end of the article, and covers what types of non-specialist products are safe to use - vinegar, lemon juice, bicarb etc. Some specialist cleaners can be run as part of a regular dishwashing cycle, but the stronger ones are best run in a dedicated cycle, in an otherwise empty machine, on the hottest setting. 

Aside from the smell, we can see quite a lot of limescale buildup in the dishwasher, so it might be necessary to run two cycles with the cleaner product. Buildup may be significantly worse in pipes & areas you *can't* see.  
Limescale is the killer of many washers & dishwashers.

A fully-functioning water softener will keep the machine limescale free & also improve the wash quality.

In warmer weather [maybe above 21°C or so indoors] you're going to need to run a cleaning product every month. Your nose will tell you when you need to do it again. Once you've re-established a regular cleaning routine, you won't need to use the expensive cleaners so often, you can get away with the cheaper in-wash cleaners.


  [1]: https://thecleanr.com/does-bleach-affect-stainless-steel/
  [2]: https://inthewash.co.uk/dishwashers/best-dishwasher-cleaners-in-the-uk/
  [3]: https://www.which.co.uk/reviews/dishwashers/article/how-to-clean-a-dishwasher-am44P4i33hzl