London - Lille - Lens?

I am traveling on the eurostar from london to lille - I am continuing from their to Lens. Is it true that you do not need an additional ticket for the local train and that you just need the eurostar ticket - is this correct?

I am pretty sure that's not true.
I live in Arras, close to Lens and I know Lille pretty well and took the eurostar to go to Londo from there a few times and I am pretty sure your eurostar ticket cannot take you to Lens. Eurostar is an independent company from the rest of french trains. You might even have to change the train station in Lille, the eurostar arrives at a station called "Lille Europe" and I'm pretty sure you have to go to the train station called "Lille Flandres" to get a train to Lens, the stations are very close to each other, definitely walking distance and a ticket between Lille and Lens can't be very expensive at all.
I hope this helps.