OK thank you for your reply and your encouragement.

I can get my code to run fine up to about n=1,000,000 but after that it times out. It does run to completion on my computer in about 20 seconds. I'm using a simple for..loop. Can anyone give me some guidance on where to look to how to improve the performance of my code for values of n between 1 million and 2 million?

I'm actually very satisifed that I have cracked the test and pretty much ticked all the boxes apart from this last one.

PS I'm using Java!