Hi my phone storage is full. If I back up on I cloud all my photos can I delete all photos on my phone to clear memory up ? I assume I can then still access them on my I pad ?

You may want to provide more information on how your photos are on both your iPhone and your iPad; i.e., how did you transfer them from one device to the other (if they are in fact already on both devices).

iCloud Photo Library is a syncing service. Any photo you delete from a device where iCloud Photo Library is turned on will also be deleted from iCloud and from any other device connected to that iCloud Photo Library. You can potentially reduce the storage space required on a device by turning on Optimize Storage. See: iCloud Photo Library - Apple Support

Apple does not currently offer cloud-only storage for photos. If that is what you want or need, you may want to explore 3rd party options such as Google Photos or Dropbox.

Another way that photos may be on iCloud (if you don’t have iCloud Photo Library turned on) is via a backup of an iOS device. The only thing you can do with a backup is use it to restore a device or set up a new device. The contents of backups are not viewable at iCloud.com and if you delete a photo from a device it will not be included in future backups and will be lost.

Read the last paragraph of what FoxFifth's reply very carefully. One way to clear the photos from your device and still have them is to archive them to a computer (and a thumb drive for safe keeping):

Transfer photos and videos from your iPhone, iPad, or iPod touch - Apple Support

Another way is a third-party solution such as Google Drive (from the App Store).