Just to confirm , you say both programes "11" and "10.2" will run on my iMac ?
They will, be sure to keep both versions. The new version does not support everything the old version could, like for example vocal transformers or podcast tracks, so there are many disappointed users out there.
But you get automatic drummers and arpeggiators with the new version, and more control over the effects - that part of GarageBand now is more like Logic.
Should I wish to un install is it just a case of moving the icon to the trash , and will that get rid of all the files for that programme or will that mess up the other
Keep the new version - it does not need much space - the most space is needed for instruments and loops, and that you will want to keep installed to be used with GarageBand '11. If you uninstall GarageBand 10.0, you will not get any updates for the instruments. And as I said, you need the new version, if you want to use the Mac version of GarageBand to open songs created on your iPad. You will not be able to use Logic Remote on the iPad to remotely control GarageBand on the MAc.
But to answer your question - moving he icon to the trash will delete the application, but let the instruments installed.