Car seat straps - UK. Please help!

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Hi all, we have the joie I-anchor advance for our daughter but have been having some issues with the straps. When the removable insert is used the straps dig into her legs and leave large red welts but without she seems to have a lot of space between her and the straps even on tightest setting.

We went to Mothercare today and two members of staff said that it needed to be used without the insert and 'it seemed to be OK and she should be fine'. Now I know they are carseat safety trained, however it's not sitting right with me. I can easily get two fingers between her and the straps and there is a lot of give. We are not going out in the car again tonight and plan to go to Halfords tomorrow to get a second opinion. In the meantime, please can people have a look at the pictures below and either confirm I am being a crazy lady or that I am right to worry?! Thanks in advance.