One page on my site started appearing in the google rankings around 3-4th place for about a week and has suddenly dropped from existence.

I thought that after 9 months of nothing from google I was finally getting somewhere then my site vanished

Any ideas?

Search term I was coming up trumps with was 'gold number sim card' for a subdir of my site. The main site was not coming up at all.

It may just be temporary, or you could have got banned. I think you will just have to wait and see, 9 months is a long time to then suddenly disappear.

Has you site completely vanished or is it just under that search term?

Probably just different datacenters