non-resident taxpayer been paying PAYE for 5 years

Just got a new client, a contractor, who's working through his limited company, taking around £12000 salary and the rest in dividends. He's been paying tax and NI, using a normal 747L (or equivalent for previous years) tax code.

However, he works on oil rigs around the world - spends maybe 3-4 weeks a year in the UK, and does no work whilst he's here. Has been in Far East, who-knows-where else, and is now in Arctic Circle (Norwegian waters). Has never worked in UK (nor UK waters)

Surely he should have been on a NT tax code, and not been paying NI (at least after the first year).

Obviously, the company is UK resident and the CT paid is correct, but could a largeish tax-free bonus be paid, and carried back against profits for the last 3 years?

How easy is it going to be to reclaim:

1) PAYE paid

2) NI paid

3) CT paid, assuming he now closes the company and claims Terminal Relief.