And I would go to halfords not mother care. I have heard mothercare give very bad advice.
Halfords often do deals on the website and have things much more expensive in store. It is worth looking first and reserving one to check fit. Some places do price match and you can always try in store and order off Internet from Kiddicare or similar. You can also easily check online if a seat will fit your car. I also look on amazon and read reviews.
If pre-loved then make sure you know how old it is. They should usually be replaced after 7 years, and if in an accident obviously.
I bought a cheap one once and threw it away. My mum also wanted to use one she used from my niece who is 23 years old now. I had to be really quite firm, and bought one for her to use. It is your child so make sure you are happy with how others will transport them.
I will stop telling you how to suck eggs now. Oops.