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Hi Ian, welcome to the forums
I don't know about the Jessops one but the Kenko one will work and you won't loose autofocus (it's an AF teleconverter).
I've never actually used a t-con myself but I think that a 2x one means you'll loose 2 aperture stops so you'll need pretty good light with your 70-300 as you'll only get f/11 at 300mm. having said that, on a bright day it shouldn't be a problem at all.
If the Tamron has a stabiliser in that will help as well, if not just try to keep your shutter speed above the reciprocal of the focal length (ie, at 300mm try to keep shutter speed above 1/300th).
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