Water level in toilet pan low after flushing and refilling - why?

The main drain had broken. Whilst awaiting repair, the drain had to be rodded through 3 times. Following the final rodding, I noticed that the water level in the toilet pan - after flushing - is not returning to the level it used to. It is about an inch lower than normal. I had the drain checked again and it seems to be clear and running normally. The toilet is filling and flushing normally - it is just after flushing the water in the pan is lower than it should be - about an inch down on normal.

At the same time, the sink (which uses the same drain system) is occasionally giving back 'unpleasant fumes.'

Both toilet and sink are located downstairs. I don't know the make and model numbers - they are both about 15 years old. The house is 60 years old and has a 'clay' drain (not plastic).