Self Employed Writer Income

I prepare the self assessment tax return for a limited company director client, who has begun to earn a small amount of self-employed income from writing children's books.  Her book contracts are fairly simple to understand - she receives a payment on signing the contract, a payment when the manuscript is delivered, and a payment when the book is published.  My question is - is it as simple as including the income in the accounts according to when the various payments are due?  

For example, she wrote one book in 2011-12.  The contract was dated 1 July 2011 read:

£500 on signing agreement

£1000 on acceptance of manuscript

£500 on publication

I assume that the initial £500 is included as turnover in 2011-12.  The manuscript was accepted on 30 April 2012 and the book was published on 31 December 2012.  So, I would expect the £1000 and the £500 for publication to be included in the 2012-13 accounts.

I am aware of the possibilities of averaging income for creators of literary works over two tax years, but this is not an issue as my client's income is very small.

I have not dealt with a writer's accounts before (and wouldn't have planned to!), so any tips would be appreciated.