Garage has reported this as something to keep an eye on
Assume it's part of the electronic handbrake?
Would LR cover it under AUC warranty?

hmm...those dont normally "crack" on their own, so probably will be put down to damage, damage is not covered. no one can really say for sure without seeing it. ( assume you mean this part )

I’m no expert, see my brakes seized on post.
Just sent my rear callipers for refurb they said seizing causes actuators to crack under load. Having said that they are only £60 on eBay. 2015 RRS DIESEL HYBRID

Mine was cracked on one side where surface corrosion of the aluminium caliper had expanded the collar the plastic epb fits over, thus effectively bursting it apart. If I had spotted it within the warranty period, I would’ve expected it to be a valid claim, as it is my car is now 6 years old. Still works ok, but it’s days are numbered…….. 2016 L494 HSE

These parts do crack. See below. Same part on the L405 as L494...

https://www.fullfatrr.com/forum/topic57937-15.html

I would just replace now and not leave it. Kev

MY2014 L405 RR Vogue SE 4.4 SDV8 in Corris Grey
MY2010 L320 RRS HSE 3.0 TDV6 in Stornoway Grey