My 3 monkeys..

your 5 yr old can go into a booster unless his/her eyes are level with the back of the booster.. In that case they can go into a adult seat.. I have rung both the police station and transport department.. The police told me one thing which was no matter what my 6yr old was to go in a booster even though he is to tall.. Transport told me he is legally able to go into an adult seat and to print off the info on their website and keep it in the car which I have done..

If you want a link to the info I printed just pm me and I will send it to you..

ps can I post the link