@grahroll has provided some good advice.
A couple of questions…
It is exactly the same error or a new one?
who identified the issue and recommended the repair what didn’t work?
If it was Bosch, it seems unusual that they would not come to tbe party since the first attempted fix was under a ‘parts warranty’, one would impgine that the second one should do as well. Not doing so would give good ground to take the matter further.
If it was the repairing agent, then I would not be paying their bill especially if it was indicated that they would repair the appliance. If they got the diagnosis wrong, then that it not your fault. I would in such cases say this to them and that you will only pay the first service labour costs if they return and repair the appliance such that it is in good working order. This approach is supported under the Australian Consumer Law where " a service must be fit for the purpose or give the results that you and the business had agreed to". If they provided a service (repair) that didn’t work,then you should not be responsible for the costs of such a repair. It is unacceptable if they try to force you to pay for failed/wrong repair work.
I would be going back to Bosch and saying that under the Australian Consumer Law you have a right to have the defect with the appliance repaired under warranty, especially when they have indicated that the appliance was still subject to a parts warranty. Indicate if they do not honour their parts warranty commitment, you will be taking the matter further. Also say that Choice/Australian Consumer Association has been advised of the problem and are interested it the outcome.
Also contact the service agent and say under the Australian Consumer Law, you are not responsible for their defective appliance repair. Indicate that you will not be paying the labour costs for a repair which did not work. If Bosch indicate they will provide the heating element under parts warranty, indicate to the service agent that when this is fixed, you will only pay for the heater fitment if this results in the appliance working and returning to a condition which is fit for purpose.
It may be best to also put the above contacts in writing so that in the future there is not dispute in what was said or agreed…and also as a record should it need to be taken further.
BTW we had a experience with our Bosch dishwasher where the service agent damaged the control board when doing a repair and tried to make us pay for both repairs. We refused for the damage the service agent caused and also refused to pay for the part. After some haggling, we ended up with a repaired dishwasher.