dirt dibbler wrote:
For a mains bathroom fan in zone 2 / 3, where should the DP isolator
be located?

In the bathroom in zone 3 (or beyond the zones), or just outside the
bathroom in an obvious place.

I have a redundant immersion heater supply running to my loft, via a
switched spur in the downstairs kitchen, If this spur is replaced with
a 3A fused switched FCU, would this location be acceptable?

IMO, no, even with appropriate labelling. In BS 7671 reg. 131-14-02
requires every motor circuit to have a readily accessible device for
switching off, and 462-01-02 says that every device for switching off
for mechanical maintenance must be suitably located in a readily
accessible position.

Anyone trying to find the switch should be able to do so easily, without
having to traipse through the house looking for it in non-obvious
places, and without having to rely on the knowledge of one particular
person.

Fan will be a humidistat type, hence no requirement to connect onto
lighting, remote pull cord etc.

I'd still feed it from the lighting circuit via a conventional 3-pole
fan isolator (with one pole unused). That gives a bog-standard
arrangement that any competent person would recognise and be able to
work on safely.

--
Andy