Okies, unless the bonnet repair comes in dirt cheap and I do both, springs it is!
So, having a search on here it seems Eibach get some positive comments, but can't really find much else. My SXi sits ~20mm lower than a standard Astra. Looking at the car on level ground the rear appears to sit lower than the front by default, possibly due to the LPG tank in the back (though I run with little petrol mostly) or possibly due to the design of the front/rear arches.
I think that to keep OEM shocks and reduce the chances of clipping the front OEM splitter driving around for work, ~40mm all round seems the best bet. I know the actual figure depends on loads of things so not expecting a dead-on 40mm blah blah. I'd be tempted to go 35mm rear/40mm front but unsure if doing this fits within budget? I reckon ~£100 is sitting for the springs, should be able to get them fitted by a qualified friend, and then get the alignment/tracking set professionally.
So, suggestions welcome - eBay appears to be a cheap place for this type of thing, though even there Eibach appear a little over budget.