Finally had some time to get dirty with Subo (as the wife calls her !! ,much better looking than the Britain's got talent winner and sounds better too) .Decided to have a clear out of the rear arches to see if I could find the root cause of the classic arch rust problem .
A lot of the old stone chip protector had fallen off or was very loose so came off really easy so stripped it all off and degreased ,surprisingly the seat belt anchor points were in good nick (common MOT failure point on these so lucky there) .Removed strut which was really easy ,soaked strut bolts in WD40 every day for 3 days helped ,top suspension mounts only light rust , the problem seemed to be where the inner arch meets the top mount there is a seam which over the years had accumulated lots of muck and had started to rust through only a small hole (5p size) so treated as best I could as it's a booger to get to and filled with liquid cavity wax ,sealed then painted over with 2 coats of Hammerite .
Finally coated with Waxoyl stone chip guard .I know this probably won't last forever but I will keep my eye on it and if rust comes back in the next year or two I will have to cut it out completely and try to weld a plate up there but it really wasn't that bad so fingers crossed.
Drivers side to do next ,I hope it's in the same condition as I was expecting the worst after 20 years of water ,muck and salt .So a bit of a tip for anyone buying an Impreza just put you hand up into the rear wheel arch and feel the outer side of where the strut goes up to see if you have got any potential problems lurking up there ,if you've got really bad rusty arches I think this might be one of the causes.