Thanks very much for the detailed reply! I'm not an audiophile, DJ or sound guy by any means so all this stuff is difficult for me...
The BEHRINGER XENYX 302USB is the mixer I am looking at...not sure if it's any good but Behringer is apparently a good brand name? I know every big name has its budget line but I'm not really ready to spend £200 or so for audio stuff. I'm not sure if the podcast will go anywhere, so up to 90-100 is what I'd be OK to spend right now, and that's what the Yeti goes for used on ebay UK.
This is one of the cheap mics I'm looking at:
http://www.ebay.co.uk/itm/400844795185? ... 1436.l2649 I'm sure it's not a great quality mic (OK, probably it's crap) but I was thinking I could upgrade to a better XLR mic later, or borrow one, while enjoying the mixer (which must be easier to use than a software mixer, I'm thinking?).
I've seen something online that indicates that Cakewalk Sonar X1 Studio can mix 2 USB inputs but can't find anything about it on their current website. Older versions of Sonar can be had for about £60 but I don't know if this supports 2 USB inputs.
I'm really surprised there isn't a device like a USB hub that combines more than 1 input. Ideally I'd like something that lets me dial down or up individual mics, for cheap

To start with, I can handle saying 'move your head back, forward, ok' to deal with individual sound volume, but I figure for the same money as a Yeti, if I can get a double-mic and mixer setup why not?