Actually Nige, for some reason, when I read your first post, a picture of a
Potterton Suprema shot into my head, not an Ideal Response.
An Ideal Response is a different kettle of fish. There are gas connections
around the heat exchanger (don't touch the banjo tubes inside the boiler).

I have seen several leaks on Respose heat exchangers, and it is always a
split (very often leaking directly into the fan which hangs off the bottom
of the cylindrical heat exchanger, which can then cause explosive ignition).
These are, in my opinion, nasty boilers, sealed pressurised system combi
boilers. You will need to isolate the water to the boiler (the isolation
valves will then probably leak once you've turned them), and drain off using
the boiler's drain-off valve (difficult to get to and use)!
Conversly, I can't remember replacing a faulty fan on a Response! I have
replaced most other bits, temperature sensors, pcbs, low water pressure
switches, flow switches, pressure relief valves, all *******s to get and and
change.

I would get an engineer in if I were you, or save up and get a boiler
replacement in the spring.