I grew up next to flass vale - it's a great spot full of common woodland species. It's in the process of being 'tidied up' at the moment which means the removal of plenty of scrub, so may be the end of garden warblers and fewer whitethroat/blackcap.
There are a couple of ways in - one next to Finbarrs by the old county hospital, and one on larches rd, off field house lane.
There's more decent birding around Aykley heads, not at all far from Flass. DLI pools can get kingfisher, the new reserve can have reeling grasshopper warbler, and there are willow tit in the woods.