Fitting a fuel filler cap such as this is not difficult - I have fitted a Monza type to my 4/4 in about 1 hour at most. So long as you put masking tape on the paint where drilling is required and are comfortable loosening and tightening jubilee clips, it is a doddle. Use some "Fairy Liquid" on the fitting to get the pipe back on! Do it yourself and you'll feel better about the mod in any event. I don't think there is any real danger to your car
