It is not required for a like-for-like replacement of an accessory. It may be prudent, but would not be required.Replacing an accessory that is broken (without the inclusion of additional wiring/circuits/new CUs) is just maintenance work, if you had to just replace a light switch would you issue a MWC?
OK, if the owner demanded it you could but I don't see the benefit or any reason to mandate it.
However we have no reason to believe that this was replacement of a faulty accessory rather than alteration of or addition to the electrical installation.
The OP says: "The timer on the immersion heater...has become faulty. I need to do a like for like replacement. 10 minute job"
It's a "like for like" replacement of a time switch. It might even be one of those time switches that comes on a pre-wired base into which the timer simply plugs in.