Well I knocked the wifi router off yesterday after posting the above and have been tethering through the phone.
It's not fibre speed of course but there is no noticable difference in general browser use.
Youtube vidoes play fine, so does BBC iPlayer, but I'm not someone who streams TV a lot.
As I thought they would the data add-ons reset overnight to give a full 5GB again today.
Thinking back I seem to recall that these were promotions of "Top up your credit by so much and we will give you XXX data ongoing." so I've now got ongoing 3GB+1GB+1GB add-ons that last for a day and are being applied each day.
That's a total of 5GB a day free, which is plenty for my usual useage.
I can take the laptop down to use the wifi in the pub for any really big downloads I want.
Looks like I'll be ditching the landline broadband and saving the cost.