Thanks for mentioning STACK Ian. This question is exactly what STACK is designed to assess.
In terms of basic algebra, I'm currently putting the finishing touches to questions which test all the algebraic skills outlined in Tony Barnard's "Map of Algebraic Manipulation".
I will be finished before Christmas. These questions will be available online to try, and the source code distributed with STACK. Each question is a template which gives randomly generated versions to students and has a full worked solution (reflecting the random numbers). This represents a significant learning resource, which I hope will be useful.
To give you some idea, attached is a screen shot from one of the "solving quadratics by factoring" problems. The second part of the question (not shown at the bottom) asks for solutions.
We've put some effort into the feedback, which you can see here. While it looks a little harsh to give 0 marks for this attempt, students can always try again. In this case we apply a "penalty" of 10% of the marks deducted for each incorrect attempt, so students who take the touble to read the feedback and act on it will end up with 0.9 marks out of 1.0. We find students really appreciate this opportunity. In any case, you can opt not to display anything about marks (grades) or to choose other kinds of marking schemes. This is up to the teacher.