Social security agreements

[tomark]

I have tried to look for an answer to this but I could not find a topic - so I feel if someone have a light
A client of ours who is currently resident of Japan, and contributing to the SIA scheme, will be soon start multiple assignments in Europe, mainly Germany, France, Belgium, Holland, Spain for the next 2 years.
The client will be UK based, and will pay through a UK agent.
Is it legitimate to pay the self-emplyed straight to the Japanese bank account?
None of the work will be done in the UK, except some visits to get instuctions or report to the manager or meetings.
We know that the Japanese SIA has in place a social security agreement for almost every European country in place to eliminate dual contributions.
The will depend on staying more than 6 months in each single country.
Will the social coverage of Japan enough? Are there compulsory schemes in Europe, like euro wide coverage or federal taxes to be accounted for?
The IR link http://www.hmrc.gov.uk/nic/japan.htm
sates this possibilty, but the contractor is not British nor resident in UK.

Any advise appreciated