When there's 5 fairly heavy guys in the car it makes some really bad scraping noises, it was so painful today coming out of a servo and hearing 2 metres worth of 'scrape' when pulling out onto the road. And going uphill the rear passengers could hear a constant light scraping noise, i'm guessing tyre's rubbing on the wheel well (235/45 17" I believe)

Short of swapping the springs - which I don't really want to as the ride is nice with just me in the car. Is there any way to increase preload or something to help stiffen the rear and keep the ride height a bit higher?

The 5 of us combined would have been about 500kg, which doesn't seem like much really. It is riding lower than it should, in the sense that even a decent bump in the road will yield a big clunk or scrape

On stock shocks & springs? Not coilovers?

Shocks in good nick? How old are the springs?

Best theory - because that's all it is remember - is maybe some coil spring rubbers. See link to image. My limited understanding is these are a stop gap in tuning your suspension spring rates.

Blurb from longacre;
Fine tune your spring rate for the perfect setup
Make quick adjustments during practice or on pit stops
Used by virtually all Sprint Cup teams in every race
1 1/4" in thickness

Link to image of said coil spring rubbers;
http://i.imgur.com/Oxktm47.jpg

But, this could get horribly dangerous as I believe it would have the same effect as bottoming out on the bump stops. Which might only occur at a full load as you've described, but still never a good thing.

Aside from this theory, I got nothing aside from getting custom spring rates. Or, maybe starting a badass BMX gang with your mates and leaving the car at home. 😝

Not sure how old the shocks and springs are. I called the place it was purchased from (when new), last suspension service in their logs was in 2003 (car is a 98), but it could have had suspension work done elsewhere. I'll have a dig through the glove box there might be a service history in there. But going off the look of the springs i'd say they are stock, and the shocks replaced at some point with Monroe GT's.
Shocks seem ok in rebound and compression dampening, it doesn't hesitate. But it's hard to judge without driving a new one

Out of curiosity how much are springs and/or shocks?

If you are positive it is your rear tyres rubbing on the guards, get the guards rolled.

New springs are about the $250-300 mark, reasonable shocks (e.g. KYBs) are around $600 for a set.

i'm not positive, but i'll have a look for rub marks when I get a chance

What I am positive about is something is scraping along driveways when I pull out of them with 4 other people in the car, not even drastically big dips. You should be able to roll a typical merc/bmw sports car over without scraping. The strangest thing is when I look under the car I can't see where its scraping, but oh hell yeah its there, very alarming sound.
Bumps such as where the road meets a bridge cause a 'thud' which sounds like suspension bottoming out

Anyone else care to comment on carrying 4 other adults and/or loads of luggage? at least to gauge if my springs have weakened significantly

if you got mud flaps still on the car, then they will most likely be the problem

Short of swapping the springs - which I don't really want to as the ride is nice with just me in the car. Is there any way to increase preload or something to help stiffen the rear and keep the ride height a bit higher?
On its own, changing the preload has no effect on the ride height. What changes the ride height is having the same load occur with a different amount of compression. Under normal conditions, it doesn't matter what springs you have, the load will be exactly the same at whatever the normal ride height is for those springs. Of course, a longer spring with the same spring rate will increase the pre-load and ride height, as would putting spacers between the springs and their carriers. But you can also get the same or greater ride height with shorter springs that have a higher spring rate. That's what you get with Lovell's CFR-31 (front) and CRR-40 (rear) springs, for example. Conversely, softer springs would require more pre-load to achieve the same ride height. Since you're having trouble scaping with a heavy load, that's not what you'd want...

Note: Changing shocks won't affect the scraping, since they control the damping, not the ride height.

Theres a couple of metal speed humps at Doncaster shopping town in one of the upper car parks which look small but have an acute angle. If i drive the KH over them at about 20k's it clears, but same speed in the KL and theres a knock. I've done that twice now... Hopefully didn't damage anything...

On its own, changing the preload has no effect on the ride height

I disagree, adding spacers above/below the springs would increase the static (when its sitting under its own weight) and loaded ride height because when the spring is reinstalled in the strut, the strut is the same physical length but will be under more tension from having the compress the springs slightly further to make them fit. More tension means there is more opposing force trying to push the strut back apart again

The effect is easily noticed on a motorcycle where the rear shock is a 'coilover' type on most motorbikes. You have an adjusting and a locking ring to adjust the amount of tension on the spring. The shock remains the same physical length, but you can compress the spring further by winding it down. Doing so will adjust the static sag and the spring rate (stiffer/softer), but not the maximum length of the shock (maximum ride height I guess)

Since these struts have no adjusting rings to compress the spring further, i'm not sure how to increase preload. Adding additional rubber rings doesn't sound safe, it could slip out?
I'm also not sure how much space I have to work with. When the shocks bottom out I don't want the spring to be binding on itself

before 'correcting' me, kindly read all of what I posted, which includes:

a longer spring with the same spring rate will increase the pre-load and ride height, as would putting spacers between the springs and their carriers.
Quoting me out of context is but a pretext for your text!

http://www.zoomsquared.com/technical-information/coilover-spring-preload-explained