Bonnet catches
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The two bonnet catches at the windscreen end are the spring striker type as found on many Triumph, Leyland, Austin & MG cars amongst others. A stiff spring with a guide pin inside is fitted to a plate fibreglassed into the bonnet, and two corresponding brackets are fitted to the scuttle panel. These incorporate a sprung release lever which is accessible from the inside of the car, so normally each side can be opened separately. Some cars, however, have a bar joining the two inside the car, and so opening one side will do both at the same time.
Adjustment of these brackets is possible to a certain degree by sliding the catches on the scuttle panel, but general alignment was set by the factory when the spring catch brackets were fibreglassed in. The only way to get the bonnet to scuttle panel gap closer or lined up better is to remove the fibreglassed in bracket and re-position it.
Height adjustment (i.e. of the bonnet in relation to the scuttle panel) is possible however. To do this, simply undo the 1/2" lock nut on the spindle (inside the spring) by using a flat screwdriver to hold the spindle while undoing the nut. Then, screw the spindle in or out to achieve the desired height, and re-tighten the lock nut. Shut the bonnet and check it's height. Repeat the process if necessary to get it set up properly.
One word of warning though - the spindles can be shortened too much after a poor adjustment, and in this position they won't fully "click" shut the sliding catch. This can mean a sudden shock when driving causing the catch to spring open. Make sure the catch "clicks" shut on each side, and it should always be possible for the bonnet to be pushed down a little after the "click" has been heard (i.e. the spindle is locked in place, but is free to travel downwards a little - and should rise back due to the spring's tension). If no click is heard/felt, check and re-adjust.
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Quick release bonnet modification
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You may have realised that there are plenty of difficult maintenance & repair jobs on your M Series which would be easier if only the bonnet could be removed easily. The bonnet frame is bonded with fibreglass to the bonnet shell, and so there's no way of unbolting that. You could unbolt the two hinge bolts, but this means having to adjust the bonnet every time it's refitted. A good modification which is fairly simple to do is to make the whole assembly come away by simply removing two large 'R' clips, and pulling apart a single wiring connector that supplies the lights. This page will give you some idea of how to do it.
This is a sketch I've prepared to show how I tackled the problem. Using the original hinges, cut off the threaded part and grind it smooth. Find some suitably sized steel tubing that fits around the stub left on the hinge. The closer the better as it will prevent vibrations (if not just annoying it could be dangerous if it can work loose). Cut the tube to a suitable length and weld on the old threaded part - either into the end of the tube or butt joint it as above (I used a capped tube). It's important to make sure that the thread length isn't altered and that the weld doesn't remove too much at the base, as this could cause lack of adjustment and an unsightly bonnet gap.
Once the two halves are finished and mate together correctly, clamp them together and drill a central hole as shown to allow a large 'R' clip (or similar) to fit through, again without the hole being too large to prevent vibration/movement. Make sure this hole is drilled to allow the 'R' clip to be inserted from above when hinge is back on the chassis (see right photo below).
Above you can see the cut down part of the hinge that remains on the chassis.
To the right is the finished quick release mechanism with the bonnet shut. Note that the 'R' clip is inserted from above (makes sense really - if it worked loose upside down it would fall out more easily)!
What about the electrics?
I decided to use a special watertight connector block which meant that corrosion wouldn't be a problem even after a few years of all year round driving. Some people choose to use a towbar connector as they're easy to buy in any motorist accessory shop, but I've found they are a bit cumbersome to neatly tuck away under the spare wheel.
The connector has a rubber joint cover which stops water & moisture getting into the joint (yellow pointer), and each wire also is shrouded with a rubber sleeve (green pointer). I chose to fit the connector here as it is out of the way and is easy to disconnect with the spare wheel removed - come on, you've got to keep fit somehow and the spare wheel is an excellent way to get a 'six-pack'!
