Virgin mortgage offer extension

Gilbert21

Hi :-)
Just after a bit of info if anyone can help! We are ready to exchange on our sale and purchase but as we are buying a new build and completion won't be until around end of March, our buyers mortgage offer from Virgin will need to be extended before they will commit to exchange. I understand this is because their mortgage offer expires beginning of March and they need to ensure they have a valid mortgage offer in place to cover completion date.
They are unable to apply for an extension to their offer until the offer is 2 weeks from expiry so I'm still playing a waiting game to get to exchange.
So my question is, how long should it take from when the extension is applied for, for it to come through so exchange can take place? Second question I suppose is who applies for the extension? Their solicitor or their broker?
TIA