I've decided to connect the LD player direct to my TV and bypass the sony amp. At present the amp is upscaling the picture but I have a feeling the panasonic TV will do a better job. I have an oval conduit burried in the wall from low level to the high level TV. At present there is only 1 HDMI cable running up it as everything else connects to the amp but I now obviously need to get a second cable for the LD player. RG8 coaxial should fit ok as its only 7mm thick but is it any good?

Not sure the brand of RG8 cable but it seems to be 50 Ohms. You want a good 75 Ohm cable, If I may suggest Blue Jeans Cable company makes a wounder full 75 Ohm cable.
You can pick up bare cable with no ends for cheap from Rawcable.com but I'm not sure on shipping to the UK on it.

before you go all the trouble running cables inside you wall , run a composite cable outside from LD player into the TV. Just temporally to see if you like it any better. you can probably try a yellow ended cable that comes with any dvd or cable box for now.

for some reason unknown, Panasonic switched to 2D comb filter on their TVs in the last few years. if it doesn't have a better comb filter but still better processing, you can disable scaling in your receiver and then your Panasonic tv will handle all scaling.

Well the cables in the wall now so am waiting on the correct adpater for the TV before I can test it.

Looking into it a bit further the video processor/scaler in the sony DTR-DH820 amp is a FAROUDJA DCDi EDGE. I'm not sure what make of processor is in the TV but if you flick it onto AV mode there is an option in the settings to turn on/off the 3d comb filter. There are also far more detailed image settings for gamma, rgb etc which hopefully combine to give a better result from the LD player. The amp doesn't have any options to turn off the or make adjustments to its video processing and there is only so much I can do with the upscaled signal at the TV without screwing up settings for SKY, HTPC etc.