I have been thinking of giving my ancient Phoenix Phantom (bought 1979) tent's groundsheet a going-over with some kind of water repellent. Advice appreciated! (and now I'll go look round the rest of the site to see if this subject has been covered before...!)

 Ah! Plenty of advice already if you pop the word 'groundsheet' into the search box. I would have thought that there would be some kind of wax/cream that you could rub into the groundsheet (on the outer side). The extra protective layer is worth considering, either on the inner side of the groundsheet, or on the outer side, but it seems that water will get between the two anyway.

 I'd think that if it were on the inside water will seep up anyway from the edges, and if underneath then rainwater will get between the two and seep up 'as before'. No easy answers. My tent has been dry as a bone in a torrential all night downpour (Oct 2006) - but found a big puddle in one corner (June 2007) during rain, with the foot of my sleeping bag in it.

 Perhaps the angle of the rain, or pitch of the surface has something to do with this. But a tip I learned is to fold sleeping bags over on themselves, and perhaps put them on top of a rucksack when leaving the tent, so (in my case) the feets are not vulnerable and they are 'high & dry'.

Fabsil apparently does the trick? Towards the bottom of the page.

Sorry to be the bearer of bad tidings, but a simple spray-on/wipe-on treatment is very probbably Not Going To Work if your groundheet is leaking.  If you want a waterproof groundsheet then get a replacement (unless you've got a serious sewing machine and are good with it I'd suggest getting a professional job done, for example at Scottish Mountain Gear.  It's doing a very different job to a fly-sheet, which only has to deal with rain, not constant pressure potentially against a large reservoir, so what will work acceptably on a fly won't necessarily work on a ground sheet.  What you'd be doing is a bit like trying to have a DWR coat on a jacket do the work of the waterproof membrane...  As delivered from the factory, a good tent fly has multiple coatings of (usually PU) waterproofing laid on in a molten state, and you can't do the same thing at home.

To check if the existing sheet really is leaking, hang the inner from a line so the whole thing is clear of the ground and pool some water into the groundsheet.  Leave it a while and inspect it a little later (and periodically thereafter) for any drips forming at the base.  If there are any, it's leaking.  It can be quite easy to think a groundsheet is leaking when all you're really seeing is condensation, but the drips through the sheet from a pool test should tell you properly whether water's actually getting through.

Pete.