Hi jaamgans, thanks for your reply.
Really the group messages should show up under the individual thread.
I agree - I'd expect my group-sent message to appear in each of the recipients' threads on my phone (as though I had sent the same text to each group member individually), and then their replies fall in after that.
Their replies do come back in the correct thread of course as you would expect, but my last sent group message isn't to be found anywhere!
I'll have a look at the alternatives in the market, but usually going down that road brings with it a whole host of new problems, when in fact I'm happy with all other aspects of the standard app. (touch wood, the "all text messages disappearing" problem hasn't happened again since updating to Gingerbread).
Now if only I didn't send so many group txts...!