3 signal not as shown in-store

I recently signed up (in store) for a SIM only deal with 3 based on the coverage checker I was shown in store - this suggested good 4G coverage indoor and outdoor where I live and where I work. When I got home I was somewhat disappointed to find intermittent signal outdoors and not a single bar indoors (upstairs, downstairs, standing on my head in the loft - you get the idea). I went back to the 3 store from which I bought the phone where the nice lady said she'd sort this out for me, I returned the SIM to her and she said she would stop the contract at no cost. I was a little disappointed and surprised when I noticed that the DD was still being paid to 3. I called them up just now to be told that if I cancelled the contract (I thought I already had) then I was liable to pay the outstanding balance of the contract. I tried to explain what the staff in the store had said, but was told I needed to go back to the store to have this discussion; the discussion I had already had! The call centre bloke also mentioned that they have a signal team who might be able to update my phone to better receive signal - I've got a HTC 10, so not the newest phone on the market, but hardly an old phone.


Questions:

1. What are my rights based on the conversation I have already had in store and the fact that the information I was given in store that turned out not to be accurate (i.e. that of the quality of signal) was the reason I opted to proceed with the purchase.

2. Has anyone had to have their phone 'updated' by a service provider in order to be able to receive a signal? What does this involve?