help! Please!

The rangie has been sat on the drive for a couple of weeks, took it for its MOT on Monday, failed on rear brake lines so brought it home and replaced them, treated it to a new battery at the same time, then started it and shows gearbox fault aswell as slow max speed 35mph...
So have gone through all the connectors to the gearbox, stripped and cleaned the fusebox which did have a bit of corrosion in, new gearbox oil filter, new fluid - the fluid was clean but the filter was very dirty, tried turning it off and on and off and on to no avail, tried disconnecting the battery and leaving it for a bit then plugging it back on, no different, the 35mph max message only comes up when the battery has been disconnected.
when you turn the ignition on you get the gear indication on the message centre saying what gear its in and it cycles through all gears fine but then when you start the engine the message "gearbox fault" comes up and the gear indication on the message centre dissapears.
It feels like the gearbox is stuck or held in 3rd, and refuses to change... Is it in some kindof safe mode?
Any ideas what I should do next? Do I need to go and get the fault cleared? My mate has a snap on reader he says he'll check what faults its logged and he can clear them... Bloody computers, why can't everything be simple like my series one!

any help appreciated, have searched and tried everything I can find... Any way I can manually reset the fault myself?

Thankyou
Jason

this may be too simple jase, but gearbox fault is one of the first messages that comes up if the battery is dodgy. i'm thinking maybe the new battery isn't fully charged / not up to the job? like i say maybe thats too simple, but its worth a go!!

i guess if you jump it with a second battery / car, it'll up the power in the electrics and you can see if the fault stays? maybe some of the more experienced heads can decide if i'm talking s@%$...

Yer thats the first thing I checked, and its fine, I thought the same, as when the battery was low it used to flick on gearbox fault so that was my first thought aswell, the battery is more than up to it its the biggest I could fit in the space, 95aH and 650A starting oomph i'll try the old one on it I suppose maybe its just having a paddy about having new bits!

right, just been and changed it back to its old battery and first time yes it fired up fine no gearbox fault then just as I drew breath to shout yipee, then beep "gearbox fault"
bloody thing! I've brought the battery in to give it a good charge maybe that will help?