The Music For Young Players pieces were intended to be a performance repertoire. Elis Pehkonen (composer of My Cats) had a percussion ensemble at Cirencester school (he succeeded Peter Maxwell Davies) that toured extensively in the UK, Europe and Australia and even appeared on Blue Peter
Brian Dennis Aquarelle:
You can hear an excerpt here:
Many of the pieces have special notations for extended effects. This is from the instruction page for EM by Vic Hoyland.
You can hear part of EM here:
You can find the full exhibiton on the Google Cultural Institute here: http://bit.ly/2oPmzJ4