After this I really must do some work!
is a Jabiru a light aircraft as such or is it a microlight?
Both versions are available in the UK, plus there is now a four-seater version available in kit form - the Jabiru J400 - which is Group A.
Does it require a CofA, or is it on the permit system?
Permit.
if I hold a PPL(A) could I fly one?
You can fly both the microlight and the Group A versions on your normal PPL. If you have the microlight one the hours won't count for your SEP renewal.
What about maintenance, who does this and where...?
You can do this yourself, but depending on what is involved you may need to get a PFA inspector to look at the results.
Can I go to the Channel Islands in it?
Yes in the GpA, no in the microlight because of an oddity in the Channel Islands law. You are meant to have a transponder, but ATC there is very understanding and will let you in without one if you ask nicely first.
are you allowed in CAS ?(Class D)...
Yes in the UK in both versions. In France you must have a transponder to enter Class D.
How much do they cost to run per year (insurance / maintenance etc....)
See the various calculations in the other thread!
Jabs are nice machines, but they are small inside. If you're tall or wide you may find them cramped. They are beautifully quiet though.