What are the pros/cons of condensing boilers? Smelly flue gasses and mist,
along with higher efficiency.

The fumes don't smell. In fact, the flue gases are much cleaner than a
traditional boiler. The only difference is that the condensing boiler fumes
are visible simply due to the water, whilst the traditional fumes stink
because they are so environmentally dirty. There are far fewer nasties in
condensing plumes.

To be honest, I don't like the ideas of newer boilers (fanned flues etc) as
all I seem to hear is "My PCB's blown again for the 3rd time, and at £150
to
replace its expensive" or "The fan's gone".

20-30% greater efficiency pays for a lot of PCBs. Choose a decent make and
you won't get these problems. Don't choose a make based on what cast iron
dinosaurs the companies made in 1965. Modern condensing boilers are almost
identical to other modern boilers in terms of complexity. The only
differences are in the heat exchanger, condensate drainage and slightly more
complicated internal programming.

Basically, the boiler is old and I want an idea of the sort of cost I
should
expect to replace it (boiler and installation),

Around 1200 +/- 200 quid for a reputable local installer (but may be
difficult to find one able to do it). Around 2500 for BG or other national.

and a recommedation for some good (reliable and efficient) boilers.

Ideal Icos
Worcester Bosch Greenstar 28HE System
Keston Celcius 25

In any case, the boiler MUST have a downward firing burner onto a single
heat exchanger. Anything less will give you grief.

Christian.