The pong is caused by bacteria in the machine. This is how I prevent my machine from smelling and it works. You may find just carrying out number 1 is enough.
1. Put a capful of disinfectant (cheapest you can find) in the fabric softener compartment. You need to do this every time you use the machine (or in the final wash of the day if you have more than one load). I've been doing this for years and since doing so, the machine has never smelled.
2. Dry the seal and leave the door open for a few hours after use, remove the detergent drawer and dry the compartment with a cloth (it takes seconds) and again, leave the drawer out for a few hours - this prevents black mould growing in there.
3. Stop using fabric softener (if you can bear to!) as it coats everything with a slimy film.
4. Avoid gel, go back to powder, I read somewhere that the machine is more likely to smell with gel.
5. Wash at 60℃ once a week or so.
Do let us know if this works for you too!