Your boiler is listed as having a balanced flue.
Extending such flues is only permitted by using the manufacturers kits and they are only designed to overcome non standard wall thicknesses. i.e. stone cottage walls.
Your conservatory company is being very iresponsible in suggesting such an alteration to the boiler outlet, so the question is; is their fitter a registered member of CORGI and is he going to issue you with a certificate stating that his work satisfies the manufacturers recommendations and the relevant H & S Gas Safety requirements. Because that is exactly what they/he should do. Your conservatory company are being crafty and devious by asking you to find or give approval to thier suggestion.
They are not going to do your electrics as well are they??
Cledlyn
February 2009