If you cannot get a key or if the locking nut is damaged by a 'Removal tool', the best thing to do is contact www.lockingwheelnutsremoved.com , They boast "never beaten".
By far the easiest way to answer this is
to advise that you get the haynes manual for your model Astra.
The subframe will need to be dropped to remove the gearbox.
I'm assuming "take it to a tire shop" is not an option for some reason. 1. Put something like Liquid Wrench on it. This includes the threads and the base. Tap with a hammer (tap, not beat), let sit...
It depends which engine is fitted. The 1.4/1.6 petrol engines require removal of the cambelt cover and cam belt to gain access to the thermostat. This in my opinion is not a job to be tackled...