I had this problem when both the rear abs sensors were nackered . . . . (due to my wheel bearings going) As soon as I changed both sensors the handbrake light went of. But I found out that when the both rear wheels have a problem with the abs it will show the handbrake with the abs warning light too. I managed to get Abs sensors for £17 online brand new.
But if its available to you get Vag-com onto it to see if its a different problem
Hope it helps