Maybe if they do a time jump after series 3 they'll recast then? Maybe Francesca too? She's such a non entity in the first series and I'm not sure if they've any reason to feature her more in the next two though I'm sure her first marriage is going to feature somewhere. Maybe if they combine/rearrange books 4-6 they'll have a few episodes to include events of the time jump years. But for Penelope and Eloise they've cast older actresses and had them play younger for series one. I've no idea about the actress playing Francesca, she seems younger. Maybe they'll keep her but I'd say they're keeping their options open.