Is it only a simple case of maintaining my altitude to within 50ft and speeds within the required limitations?
As someone already said, a lot of the CPL is about planning and preparation, but it is also about flying more accurately than before. With regard height keeping, imagine if it is a silky smooth day, then ±50ft in the cruise would be easy and the examiner may think its a poor show if you can't maintain ±20ft. However if its a rough day it won't necessarily matter if you exceed the limits laid down. Remember the examiner will be looking at the big picture so don't worry if you bust a height or speed once, but do try to be as accurate as possible all the time, especially with cruise speeds otherwise you ETAs will be wrong.
Its also much better to leave you planned cruise altitutude to remain VMC an keep the examiner informed of your intentions than to doggedly stick to planned alt in unsuitable conditions.
Personally I'd recomend hour building
after the CPL when you can consolidate new techniques and skills, rather than enforcing PPL habits etc! My flying certainly came on leaps and bounds after the CPL. I found my capacity rose dramatically and flying became much more relaxed after the course.