Right - I finally got the fixed, and it's a bit of a swine - so I'll
write it down, assuming anyone actually reads this stuff (probably me,
next time I have to fix one).

Read the whole thing AND read the quick fix in case you get lucky.

How it works:
Two halves to the machine, back half coloured blue - contains motor and
electronics, front half coloured black - contains gearbox, hammer gear
and hammer/drill&hammer/drill switch. The hammer action is entirely
mechanical, no electrics at all in the black half.

Dismantling:
Take off chuck & adjustable grip.

Electrics: To access electrics/brushes take out 3 screws holding on the
back of the rear grip. This exposes internal wiring, mostly white (make
a diagram!). Most wires are push on connectors. Release power cable
grip and it should be possible to ease the switch/direction mechanism
out of the body. Take off whatever wires are necessary for you to get
at and undo the 2 small screws holding each brush is in place. Take out
the brushes.You may find it easy to drop the mains cable off for
general ease of work.

Mechanicals: It's important to first remove the brushes first as
described above. Look at the mounting where the chuck goes. There's a
circlip holding 2 ball bearings in place, this must come off if you
want to remove the output shaft. I used 2 screwdrivers to spring it
off, and a 24mm socket to push it back on. Now undo the 4 screws going
through the black half into the blue half, the body should then come in
two with a bit of a tug.

What you'll see: The motor stator will remain in the blue half. The
motor chasis/rotor may remain with either half. The major components
are the chasis carrying the rotot and gearbox/hammer mechanism, the
output shaft/chuck mounting (the bit you just took the circlip from)
and a small shaft that couples the rotor to the output shaft (also
carrying the hammer selection mechanism). This is how I took mine apart
- before I found the potential quickfix below.

QUICKFIX: My drill had problems with the hammer action. It was caused
by the selection switch not engaging correctly with the intermediate
shaft. You MAY be able to fix it by just removing the selector switch
and hooking the mechanism back into place with a screwdriiver through
the hole. The switch is a bit tricky to remove. Rotate the switch to
hammer only, then push the red button right into the selector switch
body with a screwdriver, further than you normally would (about another
4mm).The switch will then rotate about another 45 degrees. Now lever
the switch out from directly under the red button. Look out - it will
eject the red button and spring as it comes out. Once the switch is
out, you should see a metal clip that the stud on the back of the
selector switch engages with. Mine had climbed out somehow.

It's hard to picture how the hammer mechanism works without seeing it -
let's say the selector switch acts on a lever/clip on the intermediate
shaft, sliding it back and forth affecting both the rotation engagement
and hammer engagement.

On my drill, after taking everything apart, I figured out how to remove
the hammer selection switch, reassembled everything else, then
reinserted the selector switch knowing how to get the little stud on
the back to engage with lever/metal clip on the intermediate shaft.

Everything else reassembled fairly easily as the opposite of
disassembly, apart from the circlip on the chuck mounting - which
required pushing on witha 24mm socket.

Mechanical gearbox/hammer is really well made - pity the selector
switch mechanism wasn't quite as good.