He's a black lab/mutt cross.

I've noticed recently his coat has tinges of brown in it. When I brush/comb these brown bits it seems really thick and matted.

Has anyone else experienced this problem? I get handfulls of hair out of him.

Labs have a double coat with a thicker fluffier underocat and a longer, harsher more waterproof top coat. When they moult, in darker coloured dogs, this undercoat often appears paler, dark grey or chocolatey. This is"dead" shed hair but if not groomed out it can stay matted into the "live " coat which can be itchy for the dog and prevent good air circulation to their skin. A rubber grooming glove or zoom groom type tool can work well to remove it, as can a slicker brush if you use it gently. Once the moult is over and the old, loosened hair removed the matting and the paler colour will disappear, til next time, in about six months time. Is this your dog's first adult moult?

yeah, as above

if it happens a lot it could be worth investing in an undercoat rake to pull it out

Thanks for that info. He's about seven years old and the brown patches have become more dominant over the years. I've been using a fine comb (a"gents" comb for want of a better explanantion!) and it is doing the job very well. He finds it a bit ticklish when I use it though.

Ah yes, thats referred to as "tarnish" it also happens in older black cats., some of them end up almost chocolate and it seems to happen more if they spend a lot of time outside in UV light from the sun