Could also be a mal adjusted selector switch Daz!!!

What is that? 
It's the mechanical set of contacts that the shifter operates to select a gear.. remember one on a post once, they're prone to failure if you go through a flood for example.. Sure someone will come along with better info...
DC
Actually they are not adjusable but will fill with water when submerged i.e. in a flood
They can be dismantled to dry out, but the ingress of water will probably have triggered corrosion & it is this that can cause the gearbox ECU to become confused as to what gear has been selected
Visable symptoms of water in the switch are the leds flashing like the lights on a christmas tree probably accompanied by a gearbox error message on the MID or TID
In high mileage cars, or those where the driver habitually 'plays tunes' on the gearbox, the electrical tracks in the switch wear causing similar confusion for the ECU
Most will either fit a new switch (v. expensive) or one from a Breaker
Fitting is relatively easy with the car in the air & there are 'how-to's' on the forum - Marie posted a picture showing the switch location in the past few days which is a great help