When using acrylics, my colours seem to intensify once dry and my pictures always take on a 'chocolate box' appearance that I'm not happy with. Is there any way that I can combat this.

Ususally acrylic colours dry a little duller than when they are painted. Try using fewer colours, mixing just the three primaries and using more white paint.

it is more usual for acrylic colours to dull down a little. It really depends on what you use. I use Atelier interactive paints which have many other splendid qualities. I highly recommend hitching up with the Michael Wilcox School of Colour, as revelations occur regarding choice of pigments, and colour mixing skills are enhanced from 6 tubes or a max of 12 inc white [no black].
Dullness can also be addressed. Until recently I hesitated to varnish my work - then I used Chroma Solvent Finishing Varnish [Satin]. This gets rid of the "plastic" look of acrylic work, and deeply enhances the colours - particularly the darks.. In fact - a finished painting thus treated becomes an entirely different piece of work, and I remain thrilled with the end results.