Again, lots of very useful stuff there - thank you all. Seriously looking at a 5D now, rather than a lens.
On the long lens argument (gosh, so sorry - I didn't mean to start a fight but really appreciate that you care ), I take the point on the 70-200 and will consider renting though there aren't many options locally.
I think I'd probably only use the 70-300 for outdoor candids from an unobtrusive distance. And I'm very happy to let the wifesistant lug it around.
Interesting comments on the wireless flash for the reception. I do have wireless triggers but had originally discounted it as too much faffing around, potentially intrusive and at risk of kids booting the light stands over. But I'll take them with me when I go to check the venue, see how it looks.
I would recommend not buying an original 5D right now, especially if you need it for weddings. 2005 release date, no way, get a 5D Mark II. Hands down it's worth the money for what you'll be doing and you need the reliability. You'll have no idea on shutter count on a 5D. You will notice the resolution difference, sorry. Did you really mean an original 5D?
If you already have the 70-300L, use it. Outside.