Hi Paul
I thought I had fixed this by cleaning the pre-motor filter, but the
cut-out
has started again after 10 mins use.

Don't know the Dyson inside out, but been in industrial vacuums for 25
years!

I would guess that your Dyson has a directly cooled motor, that is the
air being moved to provide the suction also provides the cooling for
the motor. Any blockage or restriction downstream of the motor will
cause overheating.

Any blockage or restriction upstream will affect the downstream and do
the same, so look very carefully at filters, bends, hose joiners etc
both in and out. Also check the fan blade it'self. If it's cracked or
damaged it won't be moving enough air to cool the motor.

Certainly the area around the motor and
base of the bin is very warm after 10 minutes use. Curiously the
brush-bar
still rotates at speed. I am not convinced the suction is as good as
it was
when new.

Again guessing, is the brush bar driven by the same motor as the fan?
If so, the motor is having to work harder, sounds like a damaged fan
blade.
Overloads are a warning. They weaken every time they trip.

Dave