Diesel fumes in cabin

Sometimes l get a smell of exhaust fumes in the cabin of my 110 TD5 CSW

It does not always happen, just from time to time. I looked at the exhaust and can't see any holes, and the manifold looks OK.
I think it is coming in through the heater. As with the fan turned off, it does not happen.

Is there a seal somewhere that needs replacing (heater to bulkhead?) I cannot remember getting diesel fumes in the cab of my 90 TD5.

Is it Diesel or exhaust fumes?
The seal for my fuel gauge leaks very slightly when the tank is full and I get a smell of diesel in the cab occasionally, especially when the tank is full and facing downhill.
I'm trying to find a suitable sealant that doesn't degrade with fuel. I've replaced the seal but think its just worn.

I think it is exhaust fumes.

Sometimes the smell is fairly strong, it's the same as the one you get outside with the engine running.

There's a foam collar that seals between the bulk head and the heater box and another between the heater box and the plastic duct under the wing, if either of these have failed it could draw engine bay gasses into the cab. There's also a drain hole in the bottom of that duct too which should have a one way valve on it.

Mine did this when the bottle seal on the rear door was knackered - might be worth a look?

If it's a TD5 (as I think yours is) has anyone made a hole in the rear floor to access the fuel pump? My TD5 used to make a lot of smells until I a) made a proper lid for it b) cured the leak around the fuel pump. I find a smear of Loctite instant gasket on both sides of the rubber sealing ring works very well and keeps my diesel in the tank where it belongs.

Take a look at the rocker cover gasket at the rear of the cover it has a disc of about 25mm which sometimes pops. This can cause oil vapour to escape which has a smell when hot.
It will need a new gasket fitting to cure it if this is the problem, also look at the engine breather to see if it is blocked or valve not working this can cause the gasket to pop