Decking / Track bed
The reason to use composite decking was a lot less maintenance with the UK weather & easy to fix to. I chose a neutral black/dark grey board to blend in with most things, but brown woodgrain is also available.
I wanted to use the 4-5met X 150mm lengths at around £7:50 per metre, but couldn't find any good deals on the longer lengths & went with the 3met X 140mm size at £4:00 per meter including fixings that i didn't need. I plan to use it length ways down the straights & 330mm cross ways around the corners in a fan shape. But have decided to create corner bends cut at angles that would create a 16 sided circle known as a "hex decagon". For each 90 degree corner i will need four segments of the 16. Now i could of gone for 8 or 12 sided, but the 16 sided suited the track i had "LGB R3" at around 485mm long. The equations for this will take ages to explain, so this link at The  Engineering Tool Box is very helpful as long as you just know the Radius of your circle. So if you wanted a complete R1 train track circle to fit on an 8 segment circle you would enter 600 radius & 8 in the segments & it will give you all the answers you need. On this calculation it will give you an angle to cut of 45deg, This has to be split in 2, so the cut on B & C will be 22.5deg on each side with a total of 45deg.

Most of this decking will be fixed underneath with 50mm stainless steel saddles/clips. 
One thing not to do with the decking is use strips of hard wood in the central slots. When the wood gets wet/damp it swells up & splits the deking "As per picture on right. "Glad i found out
on my 1st try" So i will try another good cheap method & use some cheap
plastic/nylon chopping boards to bridge the gap. These shouldn't swell up. 
Ground cover
To covers all ground beneath the track, 11 ton of granite chippings was needed at around £60 a ton inc vat. Now i thought this was ok considering my 14mm golden drive stones where £57 per ton & the granite chippings are a lot better looking & it adds some realism.