Thanks, that solved the issue where the Free Shipping was not taking priority.
However I still have the problem where if you change address after creating your basket, the shipping method does not change. This also means that the Free Shipping is applied before the address is selected, and then takes priority even when you choose an address which should override the free option.
To summarise, this is what we're trying to do:
Shipping Options 1-12 - £27.98
Various specific postcodes within UK
Shipping Option 13 - Free on orders over £100
Does not apply to above postcodes
Shipping Option 14 - £6.99
Anywhere else in the UK
If you spend over £100, you get option 13, regardless of which address you select, but we need any address from options 1-12 to override this.
If you select a UK address, you get option 14. If you then go back and change address to NI, Channel Islands, Scottish Highlands etc. it doesn't update to reflect options 1-12. This means you could trick the system into giving you cheaper postage.