UPDATE 1-Catlin sales growth slows, says 2010 may be tougher

* 9-mth gross written premiums up 10 pct vs 14 pct in H1

* Premium rates up 6 pct, growth rate unchanged vs H1

* 9-mth investment return 5.2 pct vs 2.9 pct in H1

* Expects some classes of business to be tougher in 2010

* Low frequency of large single-risk losses in Q3 (Adds details)

LONDON, Nov 5 (Reuters) - Lloyd's of London insurer Catlin (CGL.L) reported low rates of insurance claims in the third quarter but said sales growth had slowed since the middle of the year and cautioned some areas of business may be tougher in 2010.

"Whilst the market's good performance during 2009 means that conditions may be challenging for some classes of business during 2010, we believe that Catlin remains well-positioned to grow, both in terms of premium volume and underwriting profitability," Chief Executive Stephen Catlin said.

The Bermuda-based company said gross written premiums grew by 10 percent on a constant currency basis in the first nine months of this year, down from a 14 percent growth rate reported in the first six months of this year.

Insurance prices continued to rise with growth in average weighted premium rates of 6 percent in the year to the end of September, in line with the rate reported for the first six months of the year.

After some big losses on insurance claims in the first half of the year, Catlin said things had improved.

"The group experienced a low frequency of large single-risk losses during the third quarter. In addition, the 2009 Atlantic hurricane season to date has been benign," the company said in a trading statement.

Total net investment income for the first nine months of the year came in at $364 million, representing an investment return of 5.2 percent, up from 2.9 percent in the first half and a 1.5 percent drop in 2008.

(Reporting by Paul Hoskins)