My Range Rover is currently immobilised and the AA on behalf of Land Rover Assist are unable to release the transmission.

The manual - including the one that the AA have access to online - simply says:

"The recovery agent must activate the transmission park release before recovery commences. The procedure for activating the transmission park release is covered in a separate publication for service personnel." On the earlier models there was relaese handle under the cup holders...

My car broke down at 8.00pm last night and the AA have been out twice and the recovery company are also coming for a second time. Its now 3.00pm - 19 hours later.

All principally because they can't release the transmission for putting on a tow truck. And its stuck on my drive blocking others in ...

Anyone else found it that difficult! Current: L405 SVAutobiography Dynamic 2018, Defender D240 HSE 2020, Defender 110 Adventure 2015, Defender 90 XS 2012, Lightweight Ser III 1975
Waiting for: L460
Previous: L405 SDV8 Autobiography (2016 x 2, 2015 x1), 110 V8 1989, Discovery td5 ES Series II 2000

Sometimes it takes a long screwdriver/prybar to lever the gearbox release! I assume your car is on an incline? this then means you have 2+ tonnes of RR resting on the "Park pin" in your gearbox...

I have had to put a towel with a long block of wood on top across the center console to allow me to put a prybar through the gearbox release pull "thingy" to get enough leverage to release the gearbox from P!

You could try pulling/towing the weight off the gearbox before pulling the release...

It was on a slope but all four wheels chocked.

The transmission release under the bonnet did in fact work and has to be reset.

All sorted...! Current: L405 SVAutobiography Dynamic 2018, Defender D240 HSE 2020, Defender 110 Adventure 2015, Defender 90 XS 2012, Lightweight Ser III 1975
Waiting for: L460
Previous: L405 SDV8 Autobiography (2016 x 2, 2015 x1), 110 V8 1989, Discovery td5 ES Series II 2000