Gas (sulphur) type smell in cellar when it rains

Hello, I have a problem with the house I have just bought where whenever is rains, there's a very strong gas type smell in my cellar - quite like sulfur. This smell spreads to the rest of the house and makes you feel nauseous. I have had a drainsman come out who did a CCTV inspection and there's no damage to the drains that should cause any problems. He said because the house has been empty for some time, I should run a hot bath full of bleach once or twice a week then pull the plug to help clean the drains in case this is what is causing the problem. I have tried this but it hasn't solved the problem. I was at the house last night and it was raining slightly outside and the smell was very strong. I have noticed the smell is worse if the rain is quite fine. I am lost as to where to go now to get this issue sorted. Can anybody point me in the right direction? Any help would be appreciated.