1997 Peugeot 306 Fumes in cabin

Engine Mechanical problem
1997 Peugeot 306 Front Wheel Drive Manual 135000 miles

I have a Peugeot 306 Turbo Diesel 135,000 miles R reg.

There is an issue of fumes coming into the car when driving. It does not start straight away, gets bad after about 20minutes. Fumes are bad when driving under 30mph and in slow traffic.

There is a very small oil leak behind the engine, couple of drips on ground after about 2 – 3 days. The engine cover under the car has oil mixed with general dirt and has not dried out.

Also when starting the car cold after a week of being stationery there is a small amount of fumes/smoke that comes out of the bonnet. Nothing excessive and goes away after about 5 seconds.

Car runs fine and does not use an excessive amount of water. No excessive smoke or drips out of the exhaust when driving. Recently I have had the air filter and oil filter changed.

I am getting the whole exhaust replaced as the end of the tail pipe has corroded and there is not a proper exhaust exit and I think this could be the cause of fumes coming into the cabin.

I have asked if replacing the exhaust will stop fumes coming into the cabin but have not got a straight answer and they said it could also be the oil leak which at best could be the Rocker or at worst the Head Gasket.

What is the best way to solve as I cannot get a straight answer from the mechanic.