Nobody has posted what would be a reasonable estimate for a boiler replacement in these conditions. I have no idea, but my sister had a replacement condensing combi fitted in a cupboard in her kitchen (no roof work!) last year for (I think) around £3200. (So I do have an idea after all!) So maybe £4000 isn't too far out. As for adding value to the house, it's replacing a c/h boiler, not an investment in a hedge fund.