Mortgage broker didn't lock in rate.

Hello all,

I am currently remortgaging my flat as my deal is running out at the end of April. I was emailing my broker and requested to proceed with a 4.01% deal with Nationwide (same provider) for when my current deal ends. There was a possibility to start it earlier, but I wanted to see what the Bank of England was going to do with rates in March. My exact response was:

"I'd prefer to wait it out and see if anything happens with the rates in March, so please hold off starting the new deal. Will the current 4.01% deal remain locked in until end of April?"

His response on this was:
“No worries. I can lock in 4.01% to commence when your current deal ends. If there is a change in rates, we can then amend accordingly.”

I assumed all was good, and since then the USA started a war causing the Nationwide rates to increase slightly to 4.06%. Lo and behold the 4.01% was not locked in.

Do I just have to swallow this slight financial inconvenience or is this something I can battle if I want to?