Stiff steering

The steering on my 97 Griff has gone stiff. It's worse when the engine has warmed up. My local garage has checked the UJs and there's nothing obviously wrong with the PAS rack. TVRSW had a look 6 months ago & said it all seemed ok. It's gradually getting worse, I can put 10-20 degrees on the wheel and take my hands off, it'll stay with no return.
Any ideas what to check next?