I've spent several days reading web sites and posts on Part P but it's
all too conflicting.
Can anyone tell me whether the following are legal to do DIY:

In an airing cupboard in a bathroom.
Insert a (battery powered) timer between the (MCD protected) live wire
and the wall mounted, 13A fused switch connected to a 3KW immersion
heater. No wires to be added.
[Irony: if I can't do it, I won't; consuming more electricity. So much
for the ecological goals of the regulations].

Replace the flex for the above with an identical one.
[Irony: if I can't do it, I won't; leaving in place a flex with the
insulation getting very brittle.
So much for the safety goals of the regulations.
More irony: according to the regs I can't upgrade this wire.
Is a flex to a 20-year old specification as good as a recent one?]

Replace the three way valve in my CH with an identical one. Valve is
Honeywell Y Plan V4073A1039 with it's own flex, so the wiring to the
connection blocks, as such, doesn't change.

Replace the timer in my CH with a system compatible but better one,
without wiring change. E.G. Honeywell Y Plan ST6400C1003.
[Irony: if I can't do it, I won't; consuming more power, the current
timer doesn't support independant Heat and Water.
So much for the ecological goals of the regulations].

If legal, any tips/gotchas on doing the the above two tasks would be
appreciated.

In the kitchen:
Replace the hot water inlet solenoid on my Washing machine.

In case it's relevant: CH Boiler, CH Pump and Mains Distribution Unit
were upgraded (and the house was Building Control inspected ) two years
ago.