My two cents:
From
www.camerapricebuster.co.uk (bookmark this by the way

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- EOS 400D Body Only £437
- EOS 400D Lens Kit (EF-S 18-55mm) £479
A difference of £42 between the two
I sold my kit lens on ebay for £45 and I've seen them go for as much as £60 in the past so I think there's definitely no harm in having the kit lens in your bag as you can just punt it for minimal loss/maybe even make a profit if you decide it's not for you.
Now, I appreciate that if your budget is quite tight then maybe going down this route is not in your best interests as the extra £40odd quid could go nicely towards an alternative lens but I just thought I'd throw this option into the mix as you can definitely give the kit lens a trial without having to worry about losing ££ in the process
I think the Sigma (APO) would be ok for wimbledon. I saw some shots from a guy who was there last year using the canon 70-200 all at f4 and they were fine. They were all shot with a very low shutter speed (therefore minimal motion blur on the subject) and captured the action magnificently
If it's dull then you can just up the ISO a bit as D-SLRs give you plenty of leverage in this respect with minimal loss of image quality when you run the image through an appropriate noise reduction suite. Also perhaps shooting RAW would give you more options when post processing (but you'd then need to spend a few more ££ on additional CF cards

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