Mine leaks a bit too, it's a very old tiled roof and the tiles have pretty much perished. There's roofing felt on the inside tacked to the joists and that keeps the rain out.
Depends on why its leaking - if because its rusted into large holes, then you need to replace it
However if its smaller non-visible leaks. Get up on a ladder and put loads of silicon sealant on any gaps, joins and where the fixings are. This is around £3 from Wickes etc (or cheaper!)
http://www.isoflex.ie/prodshowind.asp?gr...&intext=int
See web link above .I have used this isoflex instant repair .It is in a spray can and you can use it in the wet also will spray above your head! .I find it helpful to use it when it is raining as you can actually see the leak .If it is a split or a small leak you use it neat if a larger split or hole you can get a fine mesh to use over the hole before spraying they supply some mesh with each can.
I have found it does just what it says and is brilliant even if used in the wet .I use it on composite stable roofs.
Find it works better if not only shaken well but stood in a bucket of hot warter for about 10 mins to thin it down a bit
if it's the "so many holes it needs replacing" and you don't have the money or time, Batonning down sheets of visqueen (preferrably black) using standard 50mm X 25mm batons should get you through the winter quite easily. B & Q were selling it off for a tenner a roll (30m x 4m) last week up by us.
As for the tiled one as long as you know and accept that you HAVE to replace it anyway, you could do the same trick i suppose. Or just bite the bullet and retile it.