Laws on seatbelt extenders

Madi • Pregnant with my 🌈 😊 due 17/10/16 and I'm from Cumbria, The Lake District! 🐑🙌🇬🇧

We're hopefully getting the travel system we want this weekend... I have a 1999 Ford fiesta, and a sneaky feeling the car seat isn't going to fit as fords are renowned for shot seatbelts ... does anyone know if seatbelt extenders are legal/safe and if they're not, where/how to get a longer seatbelt fitted?

Thanks in advance ladies! I've just over 5weeks to go.... talk about cutting it fine!