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As Ti mentioned, the first place to start looking is in the fuse box
where the fuel pump relay is. You'll need to use it to check the error codes.
Since you've just had someone replace the t-belt, they had to have the
intercooler out. That means they took the temp sensor out of it. I'd be
willing to suggest that they probably knocked one of the wires loose.
When you pull the codes you should see a code for a intake temp sensor.
If that sensor is not connected the ECU assumes that the incoming air is too
hot, thus the engine needs the timing backed out. Once the wire is resoldered
the problem should go away.
Later.
--ml