I went to an motor trimmer the other day to get my collapsed driver's seat fixed and I asked him the same question. He then showed me another one he is doing for another Defender. To do it properly you need to remove it and glue new vinyl in as the other has shrunk, about $110 if I remember correctly.
Mine has not sagged that badly and unless you saw a new one you would think the roof was just closer in, he said don't bother unless it gets worse.
Jeff

