Do you have the problem still ?
I'm new to the forum / Disco ownership - but I worked for years on brake system design & have seen this problem a few times.
Have you checked whether the problem can be removed by cracking a bleedscrew - as suggested by fyne1 ?
When it happens, you need to jack up the worst offending axle & confirm if the brakes drag badly or are siezed.
If yes, crack the bleedscrew & if the problem went away, you know its a problem up at the brake mastercylinder - which should not normally allow pressure to be trapped in the system. The brake dragging problem will happen because when the brakes get hot, the brake pads & fluid expand. If the expansion can not happen because the mastercylinder is sealed even when the brakes are not applied, it creates pressure in the system - automatically applying the brakes. If you continue driving, it can get to the point where the engine is not powerful enough to overcome the drag (and the brakes can actually catch fire !)
If the problem didn't go way after cracking the bleedscrew, check the pads to make sure they can slide in the caliper housing. You should be able to push them / the caliper pistons away from the disc.
If pressure was trapped / released by cracking a bleedscrew, there are 2 probable causes :
1 - the mastercylinder pistons are held forward of their "proper" position or
2 - the mastercylinder seals are "swollen" due to contamination in the fluid.
To check for "1" or "2", try pushing the pistons back into the calipers. Because the system is sealed, the pistons should not move inwards very far.
Now undo, the mastercylinder to servo retaining nuts & try to push the caliper pistons in again.
If the pistons will go all the way in, separating the servo & mastercylinder has allowed the piston to return further back to its "correct" position and reduced the pressure.
This indicates that the servo output rod is being held too far forward - either because it was not set correctly when new (unlikely since your vehicle is already 13 years old & it would have been spotted / sorted by now !)
OR because the brake pedal / pedal box is causing the servo inputrod to be held forward.
If this seems to be the cause, have a look at the pedal mechanism for problems which prevent it from returning fully.
If separating the booster & mastercylinder didn't allow the pistons to be pushed in, you probably need a new mastercylinder as the seals can not be changed easily.
Hope this helps !
Paul