yep, im farrrr away from you.
i work for the dreaded BG, most manufactures give average lifetime of 8 years so i hear, but with regular servicing and water treatment they last longer.
You say the heat exchanger has gone again, you have to ask why, do you have sludge or scale issues as these are the main reasons, was it your main h/e or hot water one.
Also i would go with a worcester or valiant boiler next time as unfortuantly with boilers you get what you pay for
Cost wise i would expect someoen to charge you approx 3-400 quid for a straight boiler swop (combi for combi).