Best child car seat 15kg+?

[Lochie]

My son is 3 yrs 2 months and I need to get a new car seat as his sister (9 months) is ready to move into his car seat. I am thinking I may as well purchase a stage 2/3 car seat, though it says from 4 yo it also says 15kg min weight which my son weighs more than.

Does anyone have any recommendations? I have just looked on halfords website and there are 16 pages of seats ranging from ?5-?500. Our budget is about ?100 if poss....if you pay more are you paying for safety or pointless extras? Our car doesn't have isofix either. Any tips appreciated :-)