Autumn did not bring the usual increase in visitor numbers for Persimmon but the housebuilder said sales volumes have remained stable in the second half of the year.The company reiterated its previous guidance of an increase in full year sales revenues of around 10% more than last year, but said total borrowings at the end of the year are expected to be less than £80m, lower than previous guidance.The group expects to increase underlying operating margin for the full year to around 8%, up from 4% in 2009.Mortgage availability remains a major obstacle for customers who wish to purchase a home, especially first time buyers who do not have the required large deposits. Further recovery in industry output and sales will be dependent upon an increase in the supply of mortgage products on appropriate terms, the company said.The company already has more than £460m of sales reserved beyond 2010 (2009: £500m) and is fully sold up for the current year.Persimmon has acquired around 4,000 new plots on 41 sites so far in the second half of 2010. "We also continue to improve the quality of our existing land bank in support of the delivery of improving underlying operating margins," the company said.