Past paper question (HELP)

It's always difficult and often dangerous to try to answer a query when you haven't provided the whole question.
There could be something in parts a, b, c or d1 which you haven't given which could be of use. How do I know?

What I can say is that the question tells you the items are in series. This means the current in them is the same.
It also means that the pds across them add to 6V, the supply voltage, assuming no internal resistance. Can I assume that? I don't have the complete question.

Now look at the graph. Can you identify two points on the graph (one on each line) that are such that they have the same current (this will be the current asked for) and the pds across the two components at that current, add to 6V?
Hint. These points are close to the centre of the graph where the two lines are just either side of 3V

Yes, that's why I linked to the past paper and wrote all of the information provided at the bottom...

Anyway, I had already noticed that, but dismissed it as being the correct approach because it appeared to give a range from 150mA to 170mA, which is not what the mark scheme wants. The mark scheme wants an answer of 165mA ± 5mA.

It is the correct approach.
It certainly doesn't give an answer within that range. Take another look.
On my graph and attempt I get a value close to that wanted by the mark scheme. It's a matter of how accurately you read the graph.

Maybe you could have explained your approach and why you considered it incorrect in your original post.

I took my range from the vertical separation between the 2 lines, at a p.d. of 3V, which gave the 160-170mA range. Could you explain why you've used the horizontal separation please? I felt that the horizontal separation was the incorrect approach, since the 2 p.d's add to a value slightly above 6V.