the scene after entering oblivion gate doesnt look anything like described in official game guide. there is no island in a sea of lava only the chapel with martin. This is my third game and all have been the same. cant complete closing the gate so cant continue the main quest any further. please help!!!!!!!!!!

You didn't enter the gate, you entered the castle.

You have to activate the firey gate not the door behind it.

This is a pretty common mistake.

You don't go through the Oblivion gate-- it's just a hoop of stone, so if you go through it, you're still where you started out. You have to go up to it and point the cursor at the sheet of flame in the middle of it. When you're close enough, the cursor will change to the door icon. Activate it just like you would any other closed door and it will take you to the Realm of Oblivion.