after some nasty banging for over a week now i couldnt believe my eyes when i jacked the car up and the top of the rear shocks what looks like 3 i think locaters or bolts have come away from the shocker so when jacked up the shocker can totally float about SCARY. they are cusco coilovers! could i jus change the backs or do the fronts need doing too??

Nasty and very dangerous.

Remove the rear seat and the suspension tops are exposed. £ nuts (14mm iirc) and it will be free. You might rlease the top mount by the single nut in the centre if you are lucky with a socket.

If just the rears are gone then just change them.
I would be tempted to remove them and fit Sti stock rear top mounts.

If they are pillow ball mounts they have to be libricated with a drop of engine oil every 2 or 3 months or they will fail like yours.

i have cusco coilovers, would it be possible to change just the bit thats perished or do i have to replace thewhole lot??

also if i have to change them would it be ok jus to put standard strut / spring/ damper on with cusco coilover stil on front? would the height be ok??

Find some stock rear top mounts (Sti are best) and you should be able to bolt them straight on instead of the knackered ones.
You can get mounts off David at Welcome to the API website (http://www.apiengines.com)

I would not mix the coil-overs and stock parts, the car might be dangerous as a result due to different spring and damping characteristics.

If all the struts are 'in doubt' find some good stock Sti units all round for the car.

not sure if back driver one was leaking though?? seemed wet around the cover