Mikes spring compressor looks the way to go if you can get hold of one.
I did mine on the bench with a 1M length of 12mm screwed rod through the whole assembly and out the top of the subframe.
I would not recomend this to anyone inexperienced as the force in the spring is scarey.
You cannot use anything larger than 12mm as it has to pass through the threaded bolt hole for the subframe mount (wrap top of your screwed rod in plastic at this point so thread is not damaged ), lubricate the screwed rod and wind up from the bottom,double nut and use large washers.
I was frightened the whole thing would let go, the spring fly out and kill me so kept secureing everything in rope as i progessed.
It works however ask anyone else who has done it this way and i gaurentee they
will confirm " its real twitchy bum stuff ...
To reiterate I do not recommend this method to anyone of a nervous disposition.