Hi.
Just for sake of completeness, if anyone else is having this issue, I solved the problem by taking a vacuum cleaner and cleaning the 4 top vents of the inverter which were clogged up with a thick layer of dust.
I thought there were 4 fans on the top, but they are just vents which were clogged up, and did not allow the hot air to escape.
Thanks for everyone's help.
Clive