The vailant I had installed the plumber could not big it up enough. He said to me he would fit any boiler I wanted but recommended that for efficiency, low faults, easy to work with. Coming from someone who is not accredited to a certain make and has no other reason to fit something good other than it will last and stop me calling him I thought that was a good choice.
Another deciding factor for me was noise the boiler is in my kitchen below bedroom and the potterton would wake me up in the winter when it came on. The vailant is whisper quiet.
Where exactly is your locale? 20 miles North of London! Town? Hertfordshire?
May know your installer!?