P.S. Christian - do they do direct replacements i.e. ground floor boiler in
the centre of the house using chimney flue, or do all modern boilers vent
directly (or nearly directly) to the outside?

You probably don't want an old open flue system. You wouldn't get all the
cost saving benefit of modern fan flued (and condensing) designs. However,
most boiler's flue systems could be shoved up a chimney. If it is
particularly long, the Keston Celcius 25 has the cheapest, as it is run with
a couple of plastic drainpipes.

A typical modern boiler can cope with an effective flue length of around 20m
(some are better, some are worse). A bend adds a metre or two to the
effective length. The flues can be run horizontally (with a slight incline
to allow condensate to drain) or vertically.

In some ways, a chimney is the ideal place to run a flue, as the plume
discharge is well away from the windows and no-one minds the steam floating
away from the terminal in quite the same way, as it is evocative of the old
coal burning days.

Christian.