Is it my imagination or is most high street womens clothing now very poor quality?

Materials seem ultra thin - tops and t.shirts especially (much of which mainly shrinks after one wash) - and jeans - which appear to look like jeans but are a poor imitation.

Or am I looking in the wrong places?

No you are spot on. Went to large shopping centre on Friday with a range of shops from low to high price. I bought nothing. M & S low to middle, is selling stuff akin to Dorothy Perkins. All the mid range shops have lowered themselves to a Primark quality.

Yep, I've noticed this too

I think you can still get the odd good quality item on the high street, but it takes a lot of time and effort to hunt them down!

I totally agree with this. I have had some items for years and washed great. Others have lost all shape after one wash even after following the instructions.

I wonder if its ever worth returning items that wash badly.

When I was in America last year I found the cheap clothing much better quality. I purchased lots of basics at Old Navy (vest tops, t-shirts etc) some as little as $2 per item and they all have stayed in great condition compared to basics purchased from the UK high street.