Where the property was built in a period for which cavity wall insulation is an option, this remains a very good investment. Although there are less grants available, it may still be possible to get help from your energy supplier (http://www.which.co.uk/energy/creating-an-energy-saving-home/guides/energy-grants/energy-grants-from-energy-suppliers/)
Thermal imaging can be a way to check whether your walls have been filled, and whether the cavity insulation has been correctly installed. Providing surveys are carried out in the correct circumstances (no residual solar heating in the bricks, no damp or windy conditions to alter the heat observed), images of cavity filled walls may show clumping or gaps in the insulation).
Neighbouring properties with (right) and without (left) cavity wall insulation