Property firm Helical Bar said it is starting to see 'much improved signs' in parts of the UK housing market but warned average property values continue to fall as rents decline."Helical has seen an improvement in value in respect of properties let on long leases to tenants of undoubted covenant strength. In particular, we are seeing positive yield movement in prime West End and City office investments," it said in company statement."However, average property values continue to fall as a consequence of declining rental values and investors need to exercise great caution," it added.Helical said it increased its annualised income by £356,000 in the quarter to 30 June 2009 after it completed 23 lettings."After two years of market uncertainty, Helical is now firmly on the front foot," said chief executive Michael Slade.