Switching to mortgage rate - do you pay each time if the bank are offering a better rate?

My fixed rate is up at the end of April and I'm going with my same bank (West Brom) and they are offering a lower rate than I was on before.

I understand when applying for the new deal you have to pay an admin fee.

If in a month's time, I check and they are offering a better deal, will I have to pay again if I want to switch again?

Thank you.