Is a bonus taxable?

I have a client who has asked this question that I am unsure of the answer, can anyone help?

There is one special situation on which I’d like your advice.

We lived in Korea throughout the tax year.   In May 2011 I was paid a special bonus of 50k. This was based upon my employer earnings and cashflow during their financial year 2010 (during which I was employed in Korea and was a non-resident for tax purposes).   Do I need to include this on my tax return? (company tax adviser has said that I should include a % based upon the fact that the original bonus scheme was set up before I left the UK).