Hi Baz,
I had a meta alarm with an immobiliser on my ER5, which I had for 3
years and no problem at all. I used to leave my ER5 in the shed for
weeks over the winter, come the spring it always had enough charge in
the battery to start.
Just make sure it's fitted by someone who know's what they are doing
and it should be ok and not drain the battery.
dave c