which ones did you use and where did you get 'em please?

Mine's Lockmaster, though I needed a particular type of master/slave
for double doors. Maco mechanisms looked good on some tilt and turn
windows I had in my old flat years ago.

I bought them off the web from some company I think in Leeds, can't
remember the name.

Just google multipoint locks. Most of these companies specialise in
supplying the replacement lock market for upvc doors, so have a huge
range of stuff, but limited knowledge of fitting requirements - as
they're aiming at drop-in replacements.

Most of the fitting effort is routing out grooves for the metal strip,
What sort of dimensions are we talking about here??

Strip rebate was something like 22mm wide and 2mm deep, then a trench
inside that of about 20mm wide and 7mm deep to give clearance for the
pushrod mechanism.

IIRC the easiest solution was multiple passes over the table saw doing
trench cuts - hence having the stile loose before assembling the door.

It's really important to get a accurate fit with no binding, but good
stiff support - in order for the mechanism to work smoothly.

and morticing recesses for each hook or bolt mechanism.
With a router or chisel & mallet?

In my case, with a morticing machine. I'm converting a chapel and made
new oak doors from scratch, so I'm geared up for that sort of thing.

The mortices might be a bit deep for a router, though you can
certainly route out part of them neatly, and then get the rest of the
depth with an auger bit.

Mmmm yeah - I was imagining clamping some 4X4s or similar each side of
the lock stile (on a ready made door) to give the router something
wider to run on - do you think that would work out OK?

Yes, that sounds a good plan.

Mine had a slave door with top and bottom bolts (operated by an up/
down handle and locked with a thumbturn in eurolock profile), and a
master door (rebated over the slave) with bolts top and bottom, 2
rollers and 2 hooks (operated by an up/down handle) and a centre bolt
(operated by a eurolock key/key and locking the handle).

Think of the strike plates for each of those items, and that's a lot
of bits to fit and get to operate smoothly. The outcome is very
elegant double handles on double doors, and good security - but the
time input was significant.

If you have the time to take the not insignificant trouble, you'll
have a great solution - but fitting conventional locks is a lot less
bother.