So, having been fed up with 10.11 restrictions (Disk Utility, etc etc) I'ver downgraded back to 10.10, however I can't access my photo library as Photos needs to be updated - is there anyway around this?
Not easily... no. You should have exported the photos from Photos before you went back. Then you could have imported them to iPhoto.
What you could do (it will be a hassle) is go to the Photos library in Finder and right click then select Show Package Contents, then navigate down through all the Masters folder and drag the images out. But if you have a lot of images, this will take forever.
Yes, you can still access iOS photos from iPhoto. I don't think any of the iCloud Photo Library stuff is there in iPhoto though if you were using that.