Thanks for the comments/ emails about the overlocker.
In the end, I went for this one.
(picture from sewingmachine.co.uk)
I picked the Janome because:
I have a Janome sewing machine that I really like, so I trust the quality
It can be used as a 2,3 or 4 thread machine
It has an easy thread lower loop
It has a built in rolled hem
It comes with an instructional DVD (very handy when you don't know what you're doing)
It comes with a project book (as above)
It didn't come with extra feet, but I was able to buy £190 worth for £50, and the machine was £30 off anyway so effectively I got twice as many feet for the same price as the Babylock
It has a retractable knife (I imagine I might sometimes want to produce an overlock stitch without trimming??? maybe??)
It has an adjustable cutting width
I can't recommend that particular website enough, by the way. I bought my sewing machine there last year and they do the best freebies I have ever seen, and it's free next day delivery, and they are extemely competitively priced anyway! What's more, within 10 minutes of getting the confirmation email (at 15.04 today), I received another email from them with the courier number confirming that it had been dispatched.
The even better thing, it didn't cost me anything. After watching me mooning over these for the past few days, Mr M said to me today "why don't you use my credit card to buy your interlocker?". Bless.