Today DS was engrossed at nursery playing with some blocks that were about the size of Duplo and in similar colours, but where Duplo has bumps that interlock these were flat with circular/cylindrical magnets so that they just kind of snapped together by magnetism.
Nursery didn't know what the brand name was. I think there may have been a little "hasbro" embossed on the bottom of some of them but can't find anything on the hasbro site. It was suggested that there was a bigger and chunkier version called Magnetic Megablocks, but I'd prefer to track these down if possible.
And so, as ever, I thought of Mumsnet, who always seem to identify things from the smallest and most inadequate of clues...