In article ,
Chris J Dixon writes:
As part of my central heating refit, I am having a new and
relocated hot water tank with economy 7 and boost immersion
heater elements. I plan to feed these via a Horstmann electronic
timer.

The circuit will have a dedicated 16A MCB (is the voting for RCD
protection or not?)

No RCD protection, but make sure your earth bonding is up to spec.

What are the requirements for further protection? I can't see
the logic in having a separate fused outlet for each element, but
local isolators at the point where the high temperature flexible
terminates seems sensible.

I don't have a clear picture of your intended setup, but
if the max load is 3kW, that sounds fine, except you want the
isolator to also isolate the controller for maintenance purposes.

Should I fit a fused spur unit upstream of the timer instead?

I would expect the instructions would indicate this sort of thing.
If the immersion heater load goes through it, then I see no point.
If it just takes a low current control supply with volts-free
relay contacts, then an FCU with 2A fuse would be a good idea.

--
Andrew Gabriel