I spent ages looking at mattresses of various prices. Ended up buying Hovag's from IKEA. Under £250, I thought they'd be junk but was fed up looking. They've been fantastic.
Haven't seen Hovag mentioned but will take a look at it.
Which? also rate Harrison Spinks Yorkshire Ortho, but they are a minimum of £700 in "the sales". I'm sure they're brilliant, but that's a hefty price tag.