I wouldn't trust myself to dabble under the bonnet, it's likely the car will never move again.
It may not be the gearbox, from reading about similar issues it seems there could be half a dozen reasons but all of them linked to MMT and costing £1000-£2000 to put right.
The car has a new battery as of last year.
The fault does happen under acceleration but not 'hard' as such. I neglected to mention that each time it has occurred the solution has been to turn off the engine, withdraw the key and then restart once all lights are off, simply turning off the engine and then restarting within ten seconds does not fix it.
Mileage is 56,000