I'm also not digging heels in: but it wasn't advice (from, eg, a plumber), but something I garnered from just reading around the issue. It probably depends on what boiler you're replacing.
We had ours done last year. Apparently after a decade or so, boilers substantially lose their efficiency, so my hunch is that you'll more than compensate for the outlay through decreased bills. We got a Vaillant (again) which cost about the same as Bozza paid. Can't tell you about cost of...