There used to be a sewage smell coming from the ejector pump.
To solve this, I put a vacuum on reverse on the vent pipe and then sprayed a soapy solution onto the top of the pump.
I watched some air bubbles come out of certain areas where the lid has either cracked or broken over the years.
That seemed to solve the problem in the short term although if you leave it for a week or so, the smell would come back but usage on a day to day basis was fine.

Now that the colder weather has appeared again with temps down to near zero overnight, the smell has come back on a daily basis in the morning, ie the water sits in the tank overnight and as soon as the pump goes in the morning, some air comes out of the leaks.

I cannot find any more air seepage holes with the vacuum method but it seems odd that the symptom has come back with the colder weather.
Could there be a negative pressure problem with the vent? Is there anyway to solve this?

I have attached a picture of the pump:
(a) lots of silicone as you can see my attempts to seal it.
(b) the vent pipe runs about 6ft the right of the pump, then 10ft to the outside, then you'll see an outside picture, where it runs 25ft up to the roof.
(c) The vent pipe looks like a 2inch pipe (6cm diameter)
(d) There is no central stack venting in this house as all wastewater runs into the ejector pump in the basement and the only venting is this pipe from the ejector pump

Pictures:
https://drive.google.com/folder/d/0B...RjaXctX2c/edit

Edit

Did a diy smoke test today. Smoke comes out from where the basin joins the concrete. Presumably this could mean a crack above the water line in the tank.

How would I know if sewage was leaking onto the concrete below?

I cannot replace the basin as it is set into the concrete.