To revitalise the query since folk appear to have run out of ideas. Just tried again, changed partition G (purely data) from primary to 'logical' and no boot up. Restore to primary, all ok.
Limited google info and what I have found suggests UEFI may be involved, which is a huge black hole to me.
Edit: I have however found this,
http://social.technet.microsoft.com/...w7itproinstall which suggests an unfixed bug in W7 and 8. Although the OP is a dual-booter (I am not) the symptoms are exactly the same.