Vauxhall Vectra potential gearbox problem

Hello All
I have a Vectra SRI done 100k miles has the F40 six speed gearbox. The other day I had problems putting into gear whilst driving. Never had this problem before. Had to pull over. Was recovered by the RAC who said the gearbox had gone. Went to the garage, the mechanic said definately the gearbox. £1800 to fix. I looked into this deeper on forums etc and found it could be a number of problems. Went back to the garage and he told me to drive it off the ramp. We did and then drove home 14 miles. The gears only became hard to get in to about 12 miles into the journey. It seems when the car warms up the gears stop working? I have noticed that the gear stick was moving when accelerating and decelerating, could this be a symtom of some thing else like realignment issues?. Any help would be much appreciated. Thank you.