Hi AuntieStella,
I'm pretty sure that they don't check numbers going in to the UK from France just your ID, so Mark wouldn't have needed to cover his tracks.
My guess would see Make using Olly's yellow scarf to lure him out of the bar in eps 1, maybe Mark took the scarf while Olly and Tony were swimming.
A guess would also see Mark planting the AA coin, maybe he knew about the pedo ring.
Something went wrong at the house where Mark kept Olly and he called on the mob to help clean it up, so now Mark owes the mob.
I also think something happened prior to eps 1, something we don't yet know, maybe Tony or his wife took a photograph that could be incriminating.
Does any one else think that the ears what Olly drew on the wall in the house look a loot bigger than those in the picture he drew, Mark has very big ears.
I think that Vincent knows the truth and maybe he saw Olly in the fastfood joint where he works which is what made him sick and not the drugs he is on.