I am about to change the timing belt on an ex disco 200tdi fitted to a 90. I have done several 300 tdi timing belts but never 200s. I watched a number of videos on youtube and it seems to be straightforward enough but have heard various complaints of snapped bolts, etc. Should I go ahead or should I tell the owner to buzz off and bother someone else? (he's a mate of mine and I service his vehicle as a return for IT-related favours I constantly bother him with).

Its not really any different, same issues with crank pulley ie they either fall off or need the proper puller and also the three bolts that go though the timing chest into the block can corrode.
The britpart kits are pretty good and come with a dayco belt and all pulleys/gaskets etc, disco 200 will need an extra pulley as they have one more than the def 200.
The 200 timing chest cover has many different length bolts so best to draw a pic on some cardboard and punch holes in for the bolts otherwise it can get very confusing, so Ive been told!

Recently replaced mine on a defender 200tdi
The bolts were stuck solid at the very top of the timing cover by the water pump so in the end i had to grind through side ways and get the timing cover repaired !
it was a pig of a job just to get the thing off

Once this was done it was easy enough to change the belt
even though it was my first time

I would also change the water pump, unless it is fairly new

Job done. I guess i was darn lucky. No seized bolts, no pulley welded to the crank, everything was nice and easy. Took 5 hours in total but that's because I took my time and since it was late i couldn't use pneumatic tools or air-powered ratchets to put covers, etc back together.