From what I remember, pruning of these should be removing any dead or diseased or weak stuff first, cutting 1 in 4 of the shoots back to ground level. They also need a lot more water than other fruits, so a good soaking every week during the growing season is essential, as well as regular feeding (which you seem to be doing with Growmore). I think, though, it's worth applying sulphate of potash around the roots in about February - sprinkle at the rate recommended on the box (although I always used to just throw a few handfuls about in the raspberry bed), but continue with regular Growmore or similar balanced fertiliser in season as well.