Kat, who is at the reception with estranged husband Alfie, takes full advantage of the free alcohol, and it's not long before she's looking for an argument.
It's poor Sonia who takes the brunt of Kat's insecurities. When Sonia flashes her a dirty look, Kat is convinced she's mocking her scars.
Regular viewers will know Kat got the scars after Alfie set their house on fire, in a dodgy insurance scam that went horribly wrong.
Convinced Sonia is staring, Kat grabs her and pushes her face into the cake.
But Sonia, like Alife, is only concerned with the amount of drink Kat is polishing off.
A source told the Daily Star: "Sonia is the maid of honour and wants the day to run as smoothly as possible for Jane. She just can't believe Kat could get so drunk.
"Alfie is also embarrassed and her behaviour makes him start to think thiunfs will never get back to how they were. Kat is all over the place – and what she does to Sonia is the final straw. She completely ruins the cake and makes a spectacle of herself."
These scenes will air during an hour-long special on BBC One on Thursday 19th February.