Can't offer advice I'm afraid - my son has his 2 yr development check next week (he turned 2 last month), so I'm hoping to discuss it with the hv then.
He has a repertoire of words, a lot of which we understand but I doubt a stranger would know what he wanted. Some words really clear (Mommy/Daddy/Nanny etc), but he has always made animal noises instead of the animal name (if we say 'duck' he just quacks back instead of saying the word!), the same with transport (we say train, he says 'choo choo'), and other words are his own interpretation (Percy is 'puppy', Gordon is 'Nooney' - spot the Thomas the Tank engine fan!).
He seems to have developed ways around not being able to pronounce - he sings theme tunes for programmes he can't pronounce, he calls my Dad 'Dave' as he can't say Grandad!
All the other kids I know are girls a couple of months older, who seem to be talking in sentences. I'm hopeful this is because they're girls or slightly older, but I'm worried there's something up, like a hearing problem or speech impediment etc.
His understanding is fine - maybe he's just lazy or stubborn?
Hope you get things sorted - sorry I can't help. Good luck!