I am replacing the blocks for the main sheet can any one tell me what ize they should be? i.e 40mm, 50mm, 55mm

Assuming a standard mainsheet of 6 or 7mm, you are looking at something like the Harken 29mm tie-ons or the 30mm Holts - I think both will take up to 8mm.

Whats is the rope i have seen people using for main sheets recently is it this stuff?

I have this (tapered) on my jib sheets but on the main I still have old fashioned plaited pre-stretched 8mm, IMHO you don't need anything hi-tech, the load on the string is never going to be much more than you can pull even if it is in the cleat and it isn't slippery to grip.

PS blocks are Harken 29mm.  Pay more for the blocks and less for the string I say.

I've heard good things about that stuff, but not £3.85pm worth of good things!! 

I think i'll stick with Marlowbraid, or whatever it's called now.

We have been using 7mm Marston with blocks for 8mm (Harken 29mm Ti-Lite or Holt 30mm - or the old 27mm white sheave ones) on both main and jib sheets. This stuff does not take up any water when wet but does go a little bit furry and square in the cleat areas. The small amount of stretch that this line has is actually a good thing on the jib sheet as it gives a little in the gusts. Two hard seasons on and its still OK.