Tribalelder ... Were you given the option to purchase more than 5 kilos extra and was the 5 kilos just to bring your allowance up to the standard 20 kilos?
I have looked on the website FAQ and whilst there is a facility to purchase extra kilos, the cost is more, albeit still cheaper than paying 12 per kilo excess baggage charge i.e. £70 instaed of £120 for an additional 10 kilos ....