with regard to beam pattern, i have retrofit xenons in mine as the standard lights were dog awful, ive had it checked on an MOT light tester and its 100% spot on perfect, no issue whatsoever with too much light or anything
 
I discovered that my headlight was out a couple of days ago and I have purchased a light bulb from halfords but not yet fitted it! It was £20 and I'm sure its a Xenon one! Doe's this mean that I will have to get another one to match the new one?

£20 is very expensive isn't it?
 
Don't get confused with replacement "Xenon" filament bulbs, and pukka Xenon (HID or High Intensity Discharge) gas disharge lights.

Pukka Xenon bulbs should never need replacing, but if they do, you'll be looking at significantly more than £20/bulb :)