Feliway might help:
http://www.feliway.co.uk/Stress-and-...Cat-Households
Also this website might have some useful advice:
http://www.understandinganimals.com/index.php
From that site, there's contact details for a cat behaviour specialist. She's UK based but there's a phone and email contact for her, so might be worth dropping her a line and asking advice:
http://www.understandinganimals.com/...haviourist.php
Again from same site a forum specifically for Cat issues:
http://www.understandinganimals.com/....php?board=2.0