A friend let me use her Tamron 1.4 converter yesterday. I put it on my 70-300 lens and was pleased with the results.

The Tamron SP 1.4x convertor is an excellent piece of glass. i have the earlier Adaptall one, way better than the SP 2x convertor.
I have to admit tho, that the High spec 1.5x DOI branded convertor is quicker to use in may case as I don't need to take the cameras adaptall mount off first...This is nearly as good as Tamron's unit.
2x is a different matter...DOI's 2x is a complete waste....which is why I took the elements out and used it as an extension tube! The Tamron SP 2x is the only one I use if a f5.6 max aperture doesn't matter (ie in bright light)

ive got a canon 350d, and a sigma 70-300dg macro lens, would i be able to get a 1.4 tele converter to join the two? and who"d be the best place to order from in the uk?

A good question. I ordered a Canon 1.4TC and was disappointed to discover that it would not fit my Sigma 150mm macro lens. I have since discovered that I can get it to fit by putting an extention tube between them. Sorry I cannot answer your question directly but this may be of some help.

Dave