Im going to install a GTEC1 and get the fueling checked at Perfect Touch.

My question is, if it turns out that it's running too lean or rich - can this be rectified? If so, how is it done?

Ask Graham for a leaner / richer fuelling chip

You have two options:

1. Ask grahamL to supply you with another chip with a tweaked fuel map to adjust things

2. Get a custom map for the car eg. unichip (£500)

Both of the above are assuming that any fuelling issues are not due to a problem with the car or incorrect modifications (unlikely to be this if your car just runs a chip).