weak signal

What sort of HD box........ SKY, freesat! Also has it alway's been this low or just gone like it recently and i assume your having picture/sound problems

sky hd , weather effects picture sometimes and quite often says no signal being received always been like it

 There could be numerous reason's for this,for example

 have you got a clear line of site i,e dish not obscured by tree's,building's e.t.c

Sub-standard cable could have been used,

Dish maybe slightly mis-aligned,

ETC,etc,etc

But as it's been like it since installation, i suspect the dish is mis-aligned....it only has to be marginally out to cause low signal. I'd get sky back to check it out.

FWIW no you can't just add a readily available signal booster like what is used for terrestrial aerial's to improve signal.You can get Satellite specific in-line and powered amplifier's but not really intended/or needed for standard Sky installation's. As i said above get sky back!

Would use ct125 cable rather than ct100 on that run length.

Less signal loss.