I found no useful detail about removing the OEM roof mount aerial, so this may help someone :
The aerial is fixed with a clamp plate which has a captive 19mm nut and a tongue which fits into a groove in the aerial body.
You can undo this nut with a 3/8" drive 19mm socket on a wobble bar.
Carefully pull the headlining down appx 3" and you will be able to slip the 19mm socket into the nut.
A fraction of a turn will be enough, as teh nut is little more than finger tight.
Pull the aerial up through the roof and undo the connectors.
Refitting is the reverse of removal
The aerial is fixed with a clamp plate which has a captive 19mm nut and a tongue which fits into a groove in the aerial body.
You can undo this nut with a 3/8" drive 19mm socket on a wobble bar.
Carefully pull the headlining down appx 3" and you will be able to slip the 19mm socket into the nut.
A fraction of a turn will be enough, as teh nut is little more than finger tight.
Pull the aerial up through the roof and undo the connectors.
Refitting is the reverse of removal