A condensing boiler is approx 98% efficent compared to about 67 - 76% for standard boilers.
Many plumbers will not fit the boiler unless they supply it , mainly for two reasons.
1, they don't get a cut
2, they will not at all get involved if it fails to work, it will be buyers problem.
Your price seems ok, you can probably get it cheaper if you shop around.
Very important thing to remember is that a plumber has to be BG registered to work on gas. It is well worth paying more to a person when you have the peice of mind that it is done correctly and safely.
Gloworm boilers are a leading brand. One last thing to be aware of is that these condensing boilers release C02 and water vapour through their flue (chimney), the exact same thing we breath out. On a cold dry you can see your breath, now mutiply that by a 1000 and that is what will be pumped out every time your boiler is on, think steam train.