1. Can I reopen the 2007/8 return and now claim the losses against the gains.
No, not least because you didn't claim the losses in 2000 (or possibly 2001). You have to claim the losses when they occur, and then carry them forward until you can use them.
3. For listed companies where the shares are nearly worthless (ie will cost more to sell than I will realise) do I still have to sell them to claim a loss?
Not necessarily. Start reading here http://www.hmrc.gov.uk/cgt/negligible_list.htm