Hello Brahim,
Sorry if I'm repeating questions that I've already asked you but ...
- Did you renew all four 'O' rings?
- After assembly did you remember to move the serrated clips on the straight pipe fully outwards?
Don't attempt to fit a larger 'O' ring to the thermostat housing. You will break it (I tried this as part of my thorough investigation of the cause of the leaks).
80 - 85 degrees is much too cool. Either the thermostat is faulty or it's not rated at 88 degrees. A genuine MG Rover thermostat is your best bet.
Simon