how do you replace a speedo sencor for a vauxhall combo 2004 ?

i can see a two wire going on to a screw in sencor front side of the gear box "is this the sencor as my speedo intermitantly stops and a dash light illuminates when speedo fails the speedo hardly comes on at all now, iv tryed taking the lead of what i beleave is the sencor on and off in case of corrosion but this seems all right and makes no differance when van is running so can someone tell me is it the sencor do i just unscrew it out and replace and whats its proper name