Thanks for this...I wouldn't usually look on Tesco website but I followed your link and saw the 20% off, plus £10 off cashmere, and I also found this code XX-CWMK which got me another £5! I ended up getting 2 cashmere jumpers (one for me, one for mum) for under £40 inc. P&P
In answer to your question, I would say their sizes are quite generous. I would guess a 12 would be fine for you. The quality of their cashmere is fine although it's obviously not as lovely as Brora or some other posh brands. I had a cashmere cardi from Tesco a few years ago and it wore really well.
I do remember reading something about how unethical cheap cashmere is once but I can't remember where I read it or exactly why it was really unethical.
Tesco returns policy looks reasonable so you could always order it and then return if if you don't like it.