I talked to the HV the other day about speech as I've worried about A's for a while. He didn't have any words till about 17 months, then managed about 6 at 18 months. Since then it's been slow progress, and at 22 months we were at about 20-30 words, definitely far below most of the other children we know his age.
She said in our area they've put the 2 year check back to 2.5 as at 2 there were so many children who weren't really speaking much but by 2.5 it had generally evened out. Up to 2 the understanding is more important than actually talking. She also said to try not to compare him to his friends who are girls, as generally they speak earlier and pronounce the words better.
A has actually managed a mini language explosion over the weekend whilst we've been away leaving him with my parents, he's suddenly got quite a few new words, including saying Daddy (although still not Mummy!) Although H was slightly crestfallen when I thought I heard him say "Daddy go away" earlier!