I'm in total agreement with Fizz and have made this point before to DP, who looked at me like I'd gone mad and backed away slowly (although he has no room to judge because he has his own theories about Fireman Sam and Norman Price, but that's another thread....)
Topsy and Tim's dad is shagging the neighbour, can't remember her name, and the mum knows but is grateful for the respite from his kinky sexual demands so doesn't care. Dad keeps her supplied with benzodiazepines and she swigs vodka from a tea cup to get through each grinding, tedious day with the pair of smug liitle fuckers.