Has any one any thoughts on Sonax extreme gel plastic restorer for the door lower plastic trim. I have tried several makes but never been satisfied with the end result. ( car plan, meguires etc. ).

use Mer.

I plan on doing my door handles with this:

https://www.amazon.co.uk/Gtechniq-Permanen...B004JNPTP0

My mirrors and lower door trim are perfect still. Landrover - turning owners into mechanics since 1948

2014 Orkney grey Freelander SD4 GS.
2004 Zambezi silver Discovery 2 Td5 (Gone)
1963 Surf blue Morris Mini Minor Super de Luxe (my little toy)

Be interested in your results with GTechniq, that's my go to product but never really seemed to work well on the door handles. 
Hung like Einstein, smart as a horse.

Maybe not bother then, didn't get good results for you? Landrover - turning owners into mechanics since 1948

2014 Orkney grey Freelander SD4 GS.
2004 Zambezi silver Discovery 2 Td5 (Gone)
1963 Surf blue Morris Mini Minor Super de Luxe (my little toy)

When I have used it previously, it's changed grey plastic to black (proper cleaning before hand is important). But oddly it didn't seem to work on the FL2 door handles, had a small effect but didn't last too long. 
Hung like Einstein, smart as a horse.

silicone spray is effective on black trim or even WD40 or turtle wax black trim wax. or even diesel ! 2013 FL2 XS.
Defender 90 300 TDi.
Defender 90 300 TDi CSW.
1964 MGB Roadster.
1944 Willys MB "Jeep" with bullet holes. (gone)
17 hand Irish Drought Thoroughbred (mostly lame)
Nagging Old Boiler.