Open Road

1 good mounting stud still attached and with good threads
2 studs snapped off flush with the spoiler
1 stud + the plastic surround fully out of the spoiler

tried to drill and tap the 2 that had snapped flush, 1 worked very well and now solid, 1 snapped the tap in the spoiler

so got a hole cutter and gently hammered it into the plastic surround and managed to get it out without breaking it.

that just left the stud + tap sticking out.
what i did not know was that studs are actually welded to a large metal plate about 10mm below the rubber mounting surface of the spoiler, so i then drilled right through it with the hole cutter.
edge of the metal plate can just be seen shining in the pictures below then got a strip of 2mm thick metal, drilled a 6mm hole and put an M6 bolt in it:
needed to make the width slightly narrower and elongate the hole so the bolt could move lengthways along the strip of metal.
then need to remove some of the rubber spoiler from just below the metal plate, so the metal strip will fit in.
then fed the metal strip and bolt into the hole, under the metal plate in the spoiler, as far as it will go one way, hence the need to elongate the hole for the bolt.
then feed the metal strip the other way so the bolt is in the middle of the strip of metal and the strip of metal is in the middle of the hole in the spoiler

so now the strip of metal is behind the metal plate and cannot pull through the spoiler as pictured below then refit the plastic surround, and do the same to the other damaged mountings then ,TEST FIT IT TO THE TAILGATE.
once happy it all fits, remove and remove the plastic surrounds and fit a nut to the bolt to stop it spinning when you bolt the spoiler to tailgate as its just a strip of metal, there should be a gap down either side, so you can fill the void in the spoiler with windscreen / rubber bond of your choice to keep the strip of meal from turning + bond in the plastic surround, fit and fully tighten the spoiler to the tailgate and let the bond set.
will post final pictures of it built up in the coming days.
hope this helps someone
cheers
Phil

Once you have your new stud plates fitted,
I would mask off the areas and then get some JB weld and fill in the area to pot the stud and plate into position.
Let it cure and you will be good to go.

NOTE make a bolt template to hold the bolt in position while the JB cures

thanks Mrmerlin, have some JB weld on order for the next one i do

removed it this morning and all now good for the next 30yrs

the white plastic ring around the studs is ideal to align the studs, and just for reference i gently tapped the hole cutter into it and they just twist out without damaging the actual rubber spoiler, so when refitted they align the stud back in the right position
cheers
Phil