right, i looked at my terrano engine, and there is just a flat metal cover with no adjustment.
on my navara engine the link below shows it plain as day.
http://i937.photobucket.com/albums/ad211/moby_one/IMGP2536.jpg
now, it is much easier if you take off the manifold, if you take off the plate, then there must be an adjustment on the underside of the plate. there has to be one, as the off boost delivery of fuel is governed by the pressure put on the diaphragm. if it is there, and it has to be, obviously the increse is done by unwinding it, to make it longer.
the other possibility is that there is no adjustment, which i doubt.
if it turns out that there is no adjustment, one option is to get a plate off of another bosh pump, they may well have some different sizes so check, but any turbo diesel with a bosch unit, golf/polo/jetta,navara,landrover200/300/2.5td to name a few should fit. it has to be a turbo diesel as the connection for the boost hose to the top plate needs to be there to increase fueling when under boost.
i am sorry i cannot be of more help at the mo, it is raining alot right now and did not fancy getting wet.
i need to do this adjustment on mine at some point, so i will do the proper how to probably at the weekend, and i will let you know when i have done it.
richard