Here we go. Just had a look at the NI38 form on the revenue website. Here's the relevant bit:

"If you are employed abroad
When you are employed abroad Class 1 NICs must be paid for the first 52 weeks you are there, if you meet the following conditions:
•your employer has a place of business in the UK, and
•you are ‘ordinarily resident’ in the UK (see below), and
•immediately before starting the employment abroad you were resident in the UK."

So, I didn't have to pay NI as my employer hasn't got a place of business in the UK. Remember it's all of the conditions that have to be satisified, not just one.

I could pay voluntary NI contributions, but I don't at the moment.