The real and more daring solution is to write 'WITHOUT PREJUDICE' at the top of the letter. This is how this is done in the US legal context.

However, if you don't feel comfortable reorganizing the letter or if your client won't accept it, the sentence should read:

"Nothing in this letter shall be construed to prejudice my client's claims against your company."

The phrase 'without prejudice' means that you may be making an offer or discussing events but you can not lose by doing so, you can only gain, and the communication cannot be used against you.

Here's a webpage on the concept of 'without prejudice': http://www.monacosolicitors.co.uk/articles/without-prejudice...

They're a little focused on the 'don't show it to a judge' aspect, whereas I focus more on the meaning of 'this doesn't constitute a waiver'. In any event, the result is the same.

Here's a similar phrase in the US Constitution: "nothing in this Constitution shall be so construed as to Prejudice any Claims of the United States, or of any particular State."

Here's a similar phrase: https://books.google.com/books?id=mCAwAAAAIAAJ&pg=PA1081&lpg...

Here's a wordreference post on 'without prejudice to'
http://forum.wordreference.com/threads/notwithstanding-and-w...