Problem with Garo RCD.

There is a Garo RCD in the consumer unit in my garage and it does not appear to be working (no power in sockets either). At present the switch is in the middle position i.e. halfway between On and Off. When I put the switch into the on position or into the off position it will not remain there and it returns to this middle position. Pressing the test button has no effect either.
Does this suggest that the RCD is faulty?

Most likely. Get an electrician to replace it, you don't want to be taking chances with RCDs.

could be a fault on wiring or an appliance plugged in and the RCD is actually doing what it is supposed to.

Don't take the chance get a sparks to look at it for you.

The middle position is the "trip" position.
To reset, push the switch down to the "off" position. (It should stay here if rcd is ok.)
Push the switch up to the "On" position to switch the circuits back on.

If the switch won't stay in the off position then in all likelihood the the rcd is faulty.

If the switch is ok in the off position but trips again when placed in the On position then
there is a fault on one or other of the protected circuits.

Call an electrician.

Thank you so much, Jetblue.
The middle position is not labelled at all, whereas the On and Off positions are labelled (I and O respectively). This is the reason I'm confused by the behaviour of the RCD.
I'd made the (incorrect) assumption that the trip position would be the same as the Off position (as is the case with every MCB I've ever dealt with), and that this middle position was a fault.

I pushed the switch down to the Off position (for the umpteenth time) but I pressed it with much more force this time. Sure enough, it clicked into that position and remained there. Then flicked it up into the On position and everything is hunky-dory again.

I'd have felt like some tool had I not read your comment and called an electrician.