Dettol Antibacterial Laundry Cleanser (Fresh Cotton) 4 x 1.5 Litre - £12 / £10.80 Subscribe & Save @ Amazon

It's a con. The *very* small print says you need to soak for 15 minutes for it to work. No washing machine soaks clothes for 15 minutes during the rinse cycle (when the fabric conditioner draw is emptied).
Get the Calgon anti bac liquid - it's more expensive but goes in with the washing powder and works as designed during the wash.

Is it though? I understand laundry detergent removes dirt, oil etc. but isn't anti-bacterial / anti-viral. Bacteria is only killed at temperatures well above a typical 30/40 degrees C. clothes wash so without some form of anti-bacterial solution, a clothes wash at lower temps isn't killing bacteria (it might be moving it around the drum and a lot flushed out at the end with the dirty water - but there will be bacteria remaining on your clothes). All I can say is that I definitely find clothes smell fresh many weeks later using this sort of stuff compared to before I started using it.

Washing machine never has any horrible or stale smell anymore either (edited)

So could it be used as a machine cleanser once in a while then?

I can't honestly say I have noticed a difference in the clothes to be fair. But I assume the action of it being in the detergent drawer and misting the inside of it when dispensed, means it does something. Even if that is just cleaning the machine in places that are hard to reach. It's honestly quite astounding the difference.
We did try just using the cleanser with no conditioner! That was not so good. So we use a capful of both. It says to use two capfuls, but after you put the conditioner in and close the drawer - the level of liquid is enough to trigger the auto-syphon that the conditioner drawers use to dispense. You can hear it emptying into the drum, and then the washing machine does its pre-drain before it starts.. spitting the stuff down the drain lol
I honestly would say give it a try at this price. We were impressed.

(If this were a HUKD for Calgon .. that would be a different kettle of dish) (edited)

We used to use the Tesco one which was much cheaper, and we could not tell the difference. Smelt nicer I thought. But it shot up in price, making my S&S cheaper.