Hi,
In the shipping listing in the checkout already display "free" when a shipping methods is "£ 0.00".
So, where exactly do you want to display this "free" ?
I am sure that it is possible to override an HikaShop view (but we have to know which one) to change the way that the shipping price is displayed when it is free.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.