Whilst I totally agree that for safety you should take it to a garage unless your 100% confident with what you are doing what you report is an absolute classic condition and not at all uncommon. Its almost certainly a localised tight brake pad or pads caused by wet weather use and then standing for a few hours with a hot pad and damp conditions (in other words you had a drive then parked up). The first vigerous use of the brakes will probably cure it but gentle application of the brakes, as many of us would do, which is the norm on wet roads would exaggerate it. To test if that's what it is go to an industrial estate or retail park type location when its out of hours and find yourself a decent stretch of empty tarmac. Drive up to 30MPH and brake with moderate force, if it still pulls try again at 40, then at 50 each time braking harder. Sitting stationary press the brake pedal as hard as you can and hold for 5 seconds, now try again at 30 and 40.
If at 40MPH you can just hold the steering wheel loosely with finger tips (be ready to grab it!) and under hard braking it does not turn more than a little one way or the other the problem is almost certainly gone.
If after doing the above test (which comes from an old BL Technical Information Bulletin to dealers back in the 70's believe it or not)the car pulls in any way then get it to a garage, slowly, as a matter of urgency. I think however you will find its a condition many of us have had over the years that's caused by 'stiction' between the brake pad backing material and the caliper body......get whoever services your car to pull the pads next service, draw file the metal sides of the pads where they run on the caliper and clean out the caliper body until there is a clearance and reassemble with Coppa-Slip or ROCOL and the problem will be gone until the next pad change....unless you get them to do the same to those from new ;-).
