Presumably when you do do put the screwdriver in the release hole, the lever does not move more than a few mms?

If that is the case it sounds like something is sticking. The first thing I would try is to get someone to rock the car backwards and forwards a few inches whilst you are trying to engage reverse and see if that frees something up.

If that doesn't work you'll need to remove the gearbox cowling to access the solenoid engagement mechanism. The four hidden clips are a bit fiddly so don't try to force it but look for full release details on line.

Richard