Well I called the emergency vets - they rang a poisons centre and called me back. willow bark can cause serious gastro problems and they said he may need support (i.e put on drip, tablets to absorb toxicity etc) - I burst into tears - my poor little one! Anyway as he was still okay in himself and hydrated (I had been giving him lectade) we decided between the vet and I, that it would probably be okay to watch him for now and bring him down if he got worse. - By lunchtime the next day he was right as rain.
He only ate a small very small amount so I dread to think what would have happed if he had eaten more. Suffice to say we have had a busy weekend digging up a tree and returfing!
In response to ooh - I think the problem was specifically with willow bark as it has similar compounds to aspirin. My dog is forever chewing the bark mulch in the borders - so far with no ill effects - it keeps him, from chewing my plants so I usually turn a blind eye. I will check to see if it is treated now however.
Thanks Oz x