Expressing the current through a dependent voltage source
Let's call the 4 Ohm resistor R1.

We can express the current through R1 (in the left-to-right direction) as

\$I(\mathrm{R1}) = \frac{1}{\mathrm{R1}}\left(V_1 - \left(V_3 - 8V_b\right)\right)\$, 

which is just what you've already done.

Now the only thing I think you're missing is that Vb can be expressed in terms of V2 and V3, so now 

\$I(\mathrm{R1}) = \frac{1}{\mathrm{R1}}\left(V_1 - \left(V_3 - 8\left(V_2-V_3\right)\right)\right)\$,

which of course you can simplify further using the usual rules.

I don't know who Mark Scheme is, but I believe he made a sign error in his solution.