It’s complex and I am still trying to get to the bottom of this myself but here is the response I got from the International, Future & Specialist Pension Centre in the UK (email: firstname.lastname@example.org)
“When calculating State Pension entitlement, countries can exchange their National Insurance records and use foreign insurance to make sure that customer meets that country’s minimum contribution conditions.
From 6 April 2016 you will need at least 10 qualifying years to receive any UK State Pension. If you do not have 10 years the reciprocal agreements will be considered to see if they can help you achieve the minimum number of years needed but the years from contributions made overseas will not count towards your new State Pension amount. You can also use NI contributions made in the UK to satisfy the minimum qualifying period to receive a Pension from an EEA country, or other country where the UK has a reciprocal agreement.
If you already have 10 qualifying years on your UK National Insurance record the reciprocal agreement will not need to be looked at.
Contributions paid in different countries are not added together to pay one single pension. Pensions are paid separately by each country that the customer has paid into.
If you’ve worked in several countries, you may have accumulated pension rights in each of them.
You’ll have to apply to the pension authority in the country where you’re living or you last worked. If you’ve never worked in the country where you’re living, your host country will forward your claim to the one you last worked in. That country is then responsible for processing your claim and bringing together records of your contributions from all the countries you worked in.
Every country has its own rules. The age that a customer can start to get their State Pension may also be different in other countries. You will need to contact the countries that you have paid contributions in, and they will advise of your Pension entitlement under their rules.”