Hi Nath -
I have and shoot (less often now-a-days) a Martin ML-10 Pioneer Longbow. It is rated at 50# (pounds) at 28" and the length of the bow is 68" with a brace height of 6 7/8" - 7 3/8".
My draw length is 30"/31" and as a result when I use the above bow I am pulling approximately 55# (pounds).
I have 18 wood (cedar shaft) arrows left and they are heavy splined arrows. Primarily, I use Easton Aluminum Shaft Arrows. I use the ones that are colored to resemble wood shafts and they are also heavy splined shafts.
In case you're interested, I used to get my Cedar Shaft Arrows from Kustom King Traditional Archery.
http://www.kustom-king.com/
I don't know if they will ship to England but they are definitely worth a look. They put out first rate products and have always been easy to deal with.
I think your groups will spread/shrink depending on how heavy a spline your arrows have (wood or aluminum). I prefer a stiff spline as it allows for less arrow deflection upon the release of the arrow and in my opinion shrinks groups. Off the top of my head I think that what I have done in the past is look at an arrow chart for the correct spline strength to match my draw length and pull weight and then go up two levels for a good, stiff, spline, strength.
Look at the site/link I provided to see if they don't have a arrow selection chart as to spline strength and bow draw weight.
Also look at Easton Arrows.
http://www.eastonarchery.com/
As for Carbon arrows, I just could never get used to using them. I have one carbon arrow. The carbon arrows are pencil thin -- at least the ones I have used have been and I just can't get used to them. Not meaning that that is good or bad. I just don't have any real experience with them.