Q: Having migrated my photos to the new programme under Yosemite I find that the original iPhoto programme is still on the computer. Is it safe to delete this ? There are some 10,000 photos so a lot of space could be saved.

Very little space will be saved by deleting it. Apple have developed a clever method where both apps share the same originals. Ultimately there won't be an issue deleting it, but it may also be wise to keep it around for a while.

The App itself takes up very little space. Your photo library takes up the bulk of the space, and its the same one Photos is using, so its not taking up any more space than it was previously. 

its the same one Photos is using, so its not taking up any more space than it was previously.

This is close but not correct - Photos and iphoto do NOT use the same library at all - the original photos are hard linked to both libraries so they are shared - each library has its own database and support files and as time goes by and Pictures are modified and deleted in one or the other they grow further apart so for example it you delete a large number of large photos from one and not the other then the originals will remain unused and unnecessary until the unused library is deleted

see Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support

Basically it is not a good idea to trash the iphoto library for a few months but if you get to the point where you really are sure you are going to use photos and if you have made many edits or deletions in Photos then trashing the iphoto library could be a large saving

LN