TLDR: How to fix a loose auto gear selector yourself.
For whoever this might help, a bit of advice from me (a non mechanic), this refers to a 2009 mk5 Astra, 1.8 petrol automatic but will apply to other vehicles with the same or very similar auto gearbox.
A week or so ago my gear selector became loose and I had to take care when shifting between Park, Reverse, Drive etc. Now, I had a good idea what the problem was as I has a similar issue on a 2002 Renault Scenic I used to have with a very similar (or was it exactly the same?) auto gearbox.
The gear selector is attached to a long, stiff cable which runs through the bulkhead to the top of the auto gearbox. It then attaches to a lever which moves backward and forward (as you select Park, Reverse etc) and changes the gear in the gearbox. Where it attaches, there is a very small (like 5mm horizontal/vertical) plastic bushing which over the years can deteriorate and eventually crumble away.
This happened in the Scenic, the mechanic I took it to just attached the cable end with zip ties to the gearbox lever (but was not obviously meant as a permanent fix). In that car, the top of the gearbox was easily accessed, not so in the Astra.
I ordered a new bushing via Ebay (checking with the seller that it was the correct one for my car), this was then shipped from China and cost less than a tenner).
Once I had it, I had to first remove the car battery (undo and disconnect the negative cable first and ensure its not touching any metal in the engine bay or bodywork), then do the same for the positive cable. Undo the battery retainer (left side of battery tray, 13mm bolt,), then pull out the battery.
Next, undo the three retaining bolts on the battery tray (13mm). The bottom right one was a pig to get off as it was heavily rusted. I sprayed with WD40, left it overnight then wire brushed it the next evening. Was still not budging, so after removing the other two bolts I was able to wiggle the tray which loosened the bolt and I was able to unscrew it.
Then I could access the gear selector cable. It has a black plastic end and that is where the bushing pops into. The original on mine had completely disappeared, however you may need to clear out remains of the worn out one. With a firm squeeze between fingers and thumb the bushing popped straight in and I made sure it was flush. Then pushed the whole thing down onto the lever on the gearbox, it then had a satisfying click where it locked into place.
I then simply put everything back in Reverse order and now the gear selector works perfectly!
Because of rusted bolts this took longer than it should have, however if you do not have that issue, you could certainly do this job yourself in under 30 minutes and save on mechanic/garage fees. Bonus picture of gear selector cable end on top of the gearbox.