My 18 plate SQ5 had gone in for the same issue...creak + clang on letting go and it was indeed the servo at fault.
I found a TPI about it and got them to do it under warranty at the time although interestingly, the invoice states around £900 for parts. They also confirmed it was part of the TPI they had too but I can't for the life of me remember what it was although I can see from my messages to them it was found on a number of forums so maybe search it out.
Cylinder (second most at £178)
Screw
Reservoir
Brake Line
Brake Line
Brake Servo (being the most at £229)
Screw
Secur.set
Sounds like you've had it for a short period of time...?
Other option is to look at specialists rather than main dealer although main dealer did say it was a fairly big job replacing it and they had only done a handful themselves on the SQ5.
There were a number of other recalls so do check those are done too - from memory, the wheel arch, corrosion and software update to door modules (51G5, 64F6, 66K8)
All the above is assuming you have the facelift model