Astra suspension strut nut. - Uncles
The front suspension on my car is making a banging noise when I corner at low speeds. I suspect the strut top bearing needs replacing. I have removed the plastic cap from the top of the strut, and found that the nut which will have to be removed after the spring has been compressed is recessed within the rubber mounting.Since the damper rod will have to be held whilst this nut is being undone, I won't be able to use a socket. Maybe a box spanner could be used. Are there any other ways to overcome this problem?
Astra suspension strut nut. - piston power
if my memory is right dosen't the astra have an alan key fitting at the top of its strut? so you could use a alan key and a ring spanner to undo it. failing that a socket and ratchet and be quick at first it should come off.
Astra suspension strut nut. - Dynamic Dave
The chrome piston doesn't fully descend into the shock absorber. Providing you grip it with something (molegrips or stilsons for example) at the higher end of the chrome piston, it won't matter too much if you damage it as the damage will go nowhere near the oil seal. If you are worried about damage at all, you can always wrap a rag around first before gripping with something.

That said, as "bigtee" mentions - normally there is something on top of the strut to grip it with anyway and to use a ring spanner to undo the nut.
Astra suspension strut nut. - Victorbox
bigtee is correct - there is a slot in the top for an allen key.