I know it's not a dedicated 35mm scanner, but the Epson Perfection 2400 has the necessary attachments and a light in the lid for doing all sorts of slides as well as regular scanning. about £140 ish IIRC.
I have the Epson Pefection 1670, around £80-£90 from Argos. Has the adapter in the lid that does 35mm negative and slides. Results are OK but not compared them to a dedicated 35mm scanner.
Depending which model you buy, the HP Scanject 3500 series flatbed scanners have a neg/slide attachment + lightbox in the lid. If you get one without, its about £50 to add it as an option.
Don't know how much better a dedicated neg scanner would be, though.
My dad has one, but I know its not a current model, but as far as I know its very good! Its a Nikon, one, with a scsi lead on it. Sorry but he is retired and for the first time ever does not appear to be online to ask the question!