Nationwide mortgage offer expiring soon, should I apply for a new product at better rates?

bannanna

Hello all
I applied to Nationwide for a 90% LTV mortgage back in November. The offer was accepted (hard credit check done end of November) and expires at the end of May 2021.
The purchase is likely still a couple months from completing due to outstanding issues at the vendor's side. I feel it will be touch and go as to whether the issues are smoothed out before the mortgage offer expires. According to Nationwide's website, they only do offer extensions for new builds - which does not apply to the property I am purchasing.
I have done a search on MSE mortgages today and the products at 90% LTV have miles better rates than before Christmas. £55 cheaper per month and over £20k less in terms of overall payment over the 25 year lifetime. Is there any reason why I should not apply for a new product? Will I be penalised in any way? If I do apply, should I wait another month to see if any better products appear? 
Grateful for any words of wisdom.