This is the lad
http://www.woodiesdiy.com/Product/Ronseal-Smooth-Finish-Super-Flexible-Filler-300ml/11674/1.10.1#.UmwO2ySHm9Y
Goes down like normal polyfilla, but sets to a slightly rubbery finish. Smooth it out well when laying, and it naturally levels/settles a bit too. The result is very firm and flexible, no cracks, ready for painting.
I'm also trying a section with grey tile grout at the moment - will post updates once it sets!