I'm glad you found a 100-400 at a price you were happy with. Out of interest can you let su know how much you paid ?
The current Sigma 150-500 does have image stabilisation (referred to as OS) and the canon mount version will work with all Canon DSLRs.
However regarding your Amazon comments, the product page (as I look at it right now) states there is £80 cashback available (and the lens is £1098.37) so it looks UK sourced to me. So long as it's from amazon.co.uk and not a fulfilled by or marketplace I doubt there would be any issues as cashback is specifically mentioned.
Non UK 100-400 lenses are cheaper than this, for instance it is £964 from panamoz and that's the cost via paypal which can be further dropped with the 5% discount for paying by bank transfer.