OK so going out on a bit of a limb here... tried not to add this post for fear of huge backlash but...just...got...to 
Any teacher who would even hope to be considered 'satisfactory' should differentiate appropriately as part of their daily/hourly classroom practise! That is without question.
Teachers have to aim their main teaching input at the middle ground in order to try and engage as many learners as possible... using quality questioning to try to reach those who do not understand and those who already know and need that bit more. Pupils who find it hard to understand are given additional support each day through a variety of means to ensure they remain motivated and do not lose confidence.
Additional activities provided for pupils who are judged to be G&T should be an attempt to achieve the same. Activities should also work on a whole class level too - in one memorable case our G&T pupils loved reporting back to the class on a discovery they made and spreading the enthusiasm! This was then picked up and really enjoyed by the whole group.
For those that are worried about 'singling out' - schools if doing their job should be assessing all pupils constantly and children progress at different rates so the register should change regularly... so it is never the same pupils every year (although some pupils do continually excel year on year). It is also not thought acceptable with my colleagues to label the pupils and say - you are G&T... and you are not!It is only right to involve parents as seek their opinion and approval for additional targets for them.
Different groups of pupils should be leaving or moving around the classroom daily for additional support or intervention strategies, small group work, outdoor classroom work, reading schemes etc. so small group movement should become a natural, rarely noticed part of classroom life!
Ideal world I know but manageable!
lovecheese I'm so sorry about the book scheme; very understandable to be frustrated by that. I'm sure you've been on the case and I wish you luck xx