Home DIY

The olde fashioned way is burnt sand mastic, which expands & contracts with the stone mullions and wooden sash.

Or just cement render the lot, and sell before the inevitable water damage.

Nah you want giant 80mm+ UPVC trim pieces siliconed on with whatever came out of the pound discount bucket at B+Q, then smeared with a gritty wet builders finger, thats the proper way to do it*

*Please don't.