Where is 15A262c? Do not see it!!

You must have run a Terminal command meant to set the catalog for Public Beta testers correctly - it looks very similar to the one for setting the Dev catalog correctly. Run this and it will fix you up:

sudo softwareupdate --set-catalog https://swscan.apple.com/content/catalogs/others/index-10.11seed-10.11-10.10-10.9-mountainlion-lion-snowleopard-leopard.merged-1.sucatalog.gz

Then restart the App Store.

That worked amazingly...many thanks!!

Had to do a full clean install this morning so I put Yosimite back on and with the release of parallels 11 I put OSX 10.11 in paralles was in the process of going the the through betas and beta 7 has gone straight from 5 to 7 at least it has saved a 2 gb download...

Insadently inorder to install parralel tools I had to install yosimite then update from that..

Install seems OK and once again this late 2012 iMac appears speedier, having opened a few applications.

Edit: In saying faster, Safari really has a new lease of life and Aperture is quick.

Yeah. That will get the developer seeds.

To get public seeds replace the word seed with beta like this:

sudo softwareupdate --set-catalog https://swscan.apple.com/content/catalogs/others/index-10.11beta-10.11-10.10-10.9-mountainlion-lion-snowleopard-leopard.merged-1.sucatalog.gz

If that doesn't work you can use the links instead:

10.11 developer beta 7 (build 15A263e)

delta:

http://swcdn.apple.com/content/downloads/51/34/031-32639/4hc9xodccun703wdwgyo2n0uaz81mnncfd/OSXUpd10.11.pkg

firmware update:

http://swcdn.apple.com/content/downloads/51/34/031-32639/4hc9xodccun703wdwgyo2n0uaz81mnncfd/FirmwareUpdate.pkg

public beta 5 (very slightly different build number to the one which was pulled - 15A262e)

delta:

http://swcdn.apple.com/content/downloads/31/55/031-32641/aibujvt0xa5y72bv8tksdoub3m5anpb0wp/OSXUpd10.11.pkg

firmware update:

http://swcdn.apple.com/content/downloads/31/55/031-32641/aibujvt0xa5y72bv8tksdoub3m5anpb0wp/FirmwareUpdate.pkg

Did resetting your catalog to the Developer Betas (as follows) fix the issue that your question was about?

sudo softwareupdate --set-catalog https://swscan.apple.com/content/catalogs/others/index-10.11seed-10.11-10.10-10.9-mountainlion-lion-snowleopard-leopard.merged-1.sucatalog.gz