Been looking into these dog waste digester systems, found a useful website showing how to make your own:
Pet Waste Composting
The special bio-activator solution that most websites recommend for dog waste composting does seem on the pricey side. From the info I can glean off the Internet, it seems to be an enzyme-based solution. On the 'make-your-own-dog-waste-digester' website it suggests using Septo-bac which is a live bacteria/enzyme product used in septic tanks to speed up digestion of wastes. I can't find on the Internet where to buy this in the UK, but similar products do seem to exist:
Septic tank treatment product, Bacteria & Enzymes
Hope some of this is helpful to folks!