You need to make sure you have covered everything for 4 marks first.
Comment on the accuracy and reliability of your results. Consider the accuracy of all your measurements and decide which part limits the overall accuracy. There is often one part which is a lot less accurate than the others. This usually decides the overall accuracy and reliability of the experiment.
Not only must you identify all anomalies but you must also give an explanation for each one.
Here you are explaining the reliability of the conclusions. By reliable we mean that anyone else could repeat your investigation and get the same conclusions, to within your accuracy. You may be 100% certain of your conclusions for a simple experiment, but with something like fieldwork there may be a variety of possible conclusions. Discuss, with reasons, the reliability of each option.
You must also think carefully about all the control variables. Were there any which were difficult to control, or impossible to measure, which may may have affected the results?
The key point here is that everything must be directed at improving the reliability of the conclusion. Always explain exactly what you need to do and why you need to do it. You won't gain 6 marks unless you are specific.
You do not get marks just for suggesting that another variable be investigated unless there is a clear link. For example:
...test other metals over the same temperature range to see if they change by a similar amount and with the same pattern to the graph. I would expect some metals to be very similar because.... while others may be different because...
Always remember to be scientific and give reasons if you want to get the highest marks.
If you suggest further experiments don't forget to include diagrams!