Last week some time, we had Jenny and Lilian at the Lower Loxley jewellery fair - Jenny noticed some like loitering around who looked familiar. Eventually , she twigged it was Stefan, and ended up telling Lilian what Stefan had told her and David in the out-of-the-way cafe about seeing someone blocking the culvert, whom he had thought was Rob, but couldn't be totally sure.
Then this week, Lilian went to Justin and Rob's office to see Justin, arriving at some point during Stefan's visit to Rob to say he wanted more hush money. We don't know exactly how much she overheard, but Rob blustered on about the man just leaving being a pushy, rude salesman, in a way that was totally credible and not blindingly obvious that he was really wound up by it to an extent even the most socially unaware person would notice, not at all.
Clearly Lilian heard enough that combined with her knowledge from Jenny, she could fill in any gaps when she told Justin about it all. Rob wouldn't have known that she would have recognised Stefan from the week before, so he wouldn't have realised all the salesman rubbish would be totally transparent - though even if she hadn't known who Stefan was, it wasn't actually believable.