hard to say if you paid too much for the fan or not to be honest, it all depends what price his usual supplier charges him, fans can vary from �90 to �200 dependant on the model of boiler. you could also say if the official potterton stockist says they are obsolete then anyone who has got a stock of them can charge what they like!
�100 to fit the fan does seem a bit steep, we normally charge �50, but we do tend to be on the cheap side. what you do have to rememeber though is you are paying for the guys knowledge in that he knew what the problem was whereas you didn't, he has also had to make 2 visit to your house, 1 to diagnose and 1 to fix, and your also paying for his corg registration, his van, his tools etc
you've certainly not been ripped off, he may be on the expensive side but ripped off is when someone says they cant fix your boiler when they can and they charge you a couple of grand for a new one, and ripped off is like the house of horrors programs when they charge you a �1000 for a washer and have a crap in your water tank to boot!