We had blocked sewers / drains which have now been cleared professionally but the smell is still coming from the toilets in the house. Can someone please advise how we can get rid of the smell ?

You might need to replace the wax rings under the toilet... the trap in the toilets and sinks are designed to block that, if those are not dry... there has to be an opening between them and the sewer for sewer gas to escape. You won't necessarily have a physical leak of water. Also check the sink drainpipes on top... they sometimes corrode..