I've tried heat, penetrating oil and an air impact wrench...

Turn the wheel in and get a long breaker bar (at least 750mm) and scaffold tube if needed.
I always seem to get better results with a long bar when they are really tight. 2012 TDV8 4.4 Westminster Santorini black
2016 Discovery 4 Landmark
2002 Defender TD5

...and I'll add my bullet point.

●Time

Penetrating oil over a good few days, and clean up as much around the Bolt head as possible to try and get the fluid in.
I've had weeks on some old bike parts, and eventually they loosened.
I suppose it depends on how soon the job needs doing? 2014 FFRR Autobiography
Corris Grey
Previously 2011 D4 SDV6 HSE Santorini Black

Try some Blaster 16-PB used it on some brake connectors that were rusted up and obviously delicate they released easily after two days soaking. 2012 tdv8 Westminster arrived Jan 19 gone Dec 23

200 TDi discovery bobtail gone
RR classic v8 bobtail gone
300 tdi Discovery 3 dr gone

https://www.screwfix.com/p/makita-dtw1002z...gLT9fD_BwE

I use one of these

BAZ8046 wrote:

Turn the wheel

This. It may not be obvious to the uninitiated, but better tool access makes a world of difference!

Great tip!

EDIT: He means the Steering Wheel  V6 Vogue - “Less is More” - less to go wrong!

soaked with gt85(doesn't dry up as quick as wd40 i found) left for a day or so and breaker bar

I think the factory caliper bolts come with loctite applied….
So you may be fighting loctite rather than / in addition to rust and corrosion

A retired mechanic neighbour swears by Plusgas as the best penetrant and it's all he uses now restoring old cars. 2012 4.4 TDV8 Vogue Santorini black ivory leather

Maybe this will help someone:

Found online some time ago.

Click image to enlarge

#2 for Plus Gas V6 Vogue - “Less is More” - less to go wrong!

Another for Plusgas, I also love the Homebrew as detailed 50/50 Acetone/ATF.

When I had my 4.2 I had some real Squeaky bum time with the caliper bolts due to galvanic corrosion. I got them out using an Ingersoll Rand 2135 to tighten them, and then a 600mm breaker bar with a slow and steady pull. This seemed to get them moving. One of them when "loose" was still pretty rough, so I made one of the old bolts into a rudimentary tap by cutting a couple of flutes with a Dremel, and used that to clean the threads on the caliper. I used Molyslip Ceramic grease- yes i know this affects Torque settings, and I didn't find a "wet Torque" figure from Brembo, but I had the car 6 years and it was always fine. Formerly happymadison1978

Liquid brake cleaner and diesel in a 50:50 mix works very well I've found.

Ironically, I also mix / keep it in an old Plus-Gas tin which stops the brake cleaner from evaporating

Jon 1992 RR Classic 3.9 efi Vogue
2014 Disco 4 HSE
===================
Both my fatties now gone...
Previous: 2011 L322 4.4 TDV8 Vogue SE /// 2002 L322 4.4 V8 HSE /// 2009 Discovery 3 2.7 TDV6 XS /// 2004 Defender 90 TD5 /// 1993 110 V8 Snatch Landrover /// 2005 Discovery 3 2.7 TDV6 SE (Aus) /// 1990 110 Isuzu 3.9 County (Aus) /// 1976 Series III Trayback (Aus)