her gas bills will be lower if she has her system properly cleaned and a new condensing combi fitted.
saunier duval are to put it mildly, carp.
to compare the bills a bit more accurately you could compare her unit use rather than cost as well as thia will allow you to compare boiler use year to year.
i recommend worcester bosch.
whats a good ball park figure for install of a BOSCH - all done in a day?
i tell you why... she's gone ahead and got 3 quotes and cheapest was £2,700 and she paying that out of her own pocket aswell
she has just gone ahead and ordered it etc without even consulting me - she may have got it for free under the warmfront scheme