Booster seat for 4-5 year old (Chalfont St Giles HP8)

We anticipate doing the occasional school run when our grandson starts school in September and he has outgrown the car seat we have.
Probably won't be needed for long and only for really short journeys so even a basic one (cushion only without a back section) will probably suffice.

Just updating this to say I am trying to reply to people who have responded but Freegle seems to be broken and won't let me type anything! If I can't get through to you I'll repost when I can and see if it works then.
Hopefully now messages have got through.