Bad smell from Washer / Dryer

You'd need to identify whether this smell is coming from the washing machine or the dryer. They're both capable of producing it.

If it's the washing machine, you can get cleaners like https://www.bosch-home.ie/en/product/cleaning-and-care/cleaning-products/for-washing-machines/00311925 which seem to work reasonably well.

Run the machine on very hot washes with towels and use a good quality powder detergent like Ariel or Persil - the normal bio version.

Unfortunately a lot of the more eco friendly detergents and ones that contain no enzymes don't remove biofilms from inside machines.

If you're in a hard water area, you can also get build up of limescale in the machine, which can make it easier to gunk up. In that case, a washing machine cleaner product will descale the machine.

If the smell is coming from the dryer, you may have a more complicated problem to solve.

Heat pump dryers are relatively new technology, and a lot of the problems have been ironed out mostly by the more expensive manufacturers like Miele and BSH (Bosch, Siemens, NEFF) etc and so on have been working on this for >20 years. The tech only really gone mass market in a big way in the last 5-10 years, and some of the designs are just not great.

Heat pump dryers do not get very hot. They work like an air conditioner or dehumidifier in a closed loop. There's a unit in the bottom which contains a heat exchanger that's connected to the evaporator and condenser side of a refrigeration cycle. The hot air from the drum is drawn into the cold side of the condenser, the water drops out, and then it's reheated using the same heat that was removed from it and returned to the drum again. The heat pump also draws some heat from the environment to heat the air to a decent temperature, but it's never very very hot.

They are not traditional 'condenser' dryers which used air-to-air condensers and often ran VERY hot (to the point they weren't great for clothes). Heat pump dryers don't even contain heating elements.

If the condensate system is blocked or if there's some issue where somehow bacteria and lint is stuck in there it has the potential to get quite smelly.

The only solution to that is usually to flush out the condenser and that's really only something that can be done by a maintenance technician. I wouldn't suggest trying it yourself, as you'll likely wreck the machine.

So basically identify what the issue actually is first - it could be either machine.

Washer-dryers are generally good washing machines and disastrous dryers that barely meet the definition of a dryer at all.

There's a significant issue though with detergent. One of the problems is that the UK and by overspill, as we have the same products, Ireland, are very fixated on non-bio detergent. It all goes back to a 1970s tabloid drama over Persil Bio. Basically, Proctor and Gamble had launched Ariel which was one of the first synthetic detergents with enzymes, and it worked very well. Lever Bros had been producing fairly traditional Persil which wasn't even suitable for front-loaders, which were only starting to become popular. Then they launched a 'Persil Automatic' and some of the early versions of it seem to have caused a stir due to causing allergies.

They then launched Persil Non-Bio, which was just the old formula, as a PR clean up basically. The result was that people started to associate Bio with allergies and the UK and Ireland market became uniquely fixated on the idea that non-bio is hypoallergenic / sensitive. In reality they're just old chemical based formulations that don't necessarily work very well and they aren't sold in other markets - they usually just have "Sensitive" or "Free and Clear" which usually means no scents and allergens added, but they do contain enzymes. A lot of our non-Bio products are highly scented, and not necessarily very hypoallergenic at all.

Anyway, to cut a long story short, various analyses, mannanases, glycosidases, lipases and so on in normal detergents are extremely good at breaking down proteins, starches and lipids, which means they tend to flush away biofilms very effectively. Non-bio is relying just on just the chemical components : surfactants, squestrants and bleach (peroxide) etc and just isn't as capable of breaking stuff down. The liquid and capsule versions don't contain bleach btw, so are even less likely to remove biofilms.

People using non-bio, short, luke warm washes and fabric softener is probably the main reason why this part of the world is extremely prone to having this issue.