The other day I discovered a dent in my 'newly' painted bonnet that came from underneath. It also split the paint - not happy at all.
It seems that the RH edge of the air filter housing had hit the bonnet due to failure of the RH engine mount (the one up high, at the end of the cylinder head). Colin Stark tells me this is a reasonably common failure. He sent me a new one overnight (thanks Colin).
This is worth checking every now and then to avoid bonnet damage at least. Just put a trolley jack under the RH end of the sump (with a wood packer to prevent sump damage). Apply a little pressure to see if the mount pulls apart. Obviously you don't want to lift the entire car this way or you'll probably wreck a mount that was previously OK!
I made up a special tool for unscrewing and re-torqueing the mount (possibly unnecessary) but if anyone wants to borrow it they can. Use my email or ring (I'm in PCCV) because I can go weeks (or even months - gulp) without checking this forum.