Wow, thanks everyone for your amazing feedback! Sounds like you're all in nappy heaven!
Raven here's what we're currently doing for every wash, which just gets the nappies clean - ie no smell:
Rinse each of the 3 year old's nappies by hand as we take them off her
We use paper liners in every nappy.
Dispose of poo in the loo & rinse off any eccess
Pre-rinse in the machine
Set the machine on heavy duty, 60degrees, high water level. Trick the machine into putting twice the water it intends too in. This cycle does it's own pre-rinse on top of the one mentioned above, and lasts about 2.5 hours.
Do an extra rinse at the end of the heavy duty cycle.
We sometimes also have to wash the pooey nappies entirely separately.
I think I'll definitely be getting a new machine!
Ali.